<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.8.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1063</ContextCount>
  <ElementCount>620</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>187</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>12</UnitCount>
  <MyReports>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - Consolidated Balance Sheets  (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Consolidated Balance Sheets  (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - Consolidated Balance Sheets  (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ConsolidatedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Consolidated Balance Sheets  (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - Consolidated Statements of Income  (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ConsolidatedStatementsOfIncomeUnaudited</Role>
      <ShortName>Consolidated Statements of Income  (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - Consolidated Statements of Comprehensive Income  (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ConsolidatedStatementsOfComprehensiveIncomeUnaudited</Role>
      <ShortName>Consolidated Statements of Comprehensive Income  (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004000 - Statement - Consolidated Statements of Stockholders' Equity  (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ConsolidatedStatementsOfStockholdersEquityUnaudited</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity  (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1005000 - Statement - Consolidated Statements of Cash Flows  (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ConsolidatedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows  (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Business and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Business and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Discontinued Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperations</Role>
      <ShortName>Discontinued Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Loans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/Loans</Role>
      <ShortName>Loans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Allowance For Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/AllowanceForLoanLosses</Role>
      <ShortName>Allowance For Loan Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecurities</Role>
      <ShortName>Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/Borrowings</Role>
      <ShortName>Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstruments</Role>
      <ShortName>Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValue</Role>
      <ShortName>Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquity</Role>
      <ShortName>Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Regulatory Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/RegulatoryCapital</Role>
      <ShortName>Regulatory Capital</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Commitments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/Commitments</Role>
      <ShortName>Commitments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/Contingencies</Role>
      <ShortName>Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Certain Relationships and Related Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/CertainRelationshipsAndRelatedTransactions</Role>
      <ShortName>Certain Relationships and Related Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Business Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessSegmentInformation</Role>
      <ShortName>Business Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - Goodwill</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/Goodwill</Role>
      <ShortName>Goodwill</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Business and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Business and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2301302 - Disclosure - Business and Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Business and Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - Discontinued Operations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsTables</Role>
      <ShortName>Discontinued Operations (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperations</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - Loans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansTables</Role>
      <ShortName>Loans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/Loans</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Allowance For Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/AllowanceForLoanLossesTables</Role>
      <ShortName>Allowance For Loan Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/AllowanceForLoanLosses</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecurities</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BorrowingsTables</Role>
      <ShortName>Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/Borrowings</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Derivative Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</Role>
      <ShortName>Derivative Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstruments</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueTables</Role>
      <ShortName>Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValue</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Stockholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquityTables</Role>
      <ShortName>Stockholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/StockholdersEquity</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - Regulatory Capital (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/RegulatoryCapitalTables</Role>
      <ShortName>Regulatory Capital (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/RegulatoryCapital</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Commitments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/CommitmentsTables</Role>
      <ShortName>Commitments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/Commitments</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Business Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessSegmentInformationTables</Role>
      <ShortName>Business Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/BusinessSegmentInformation</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2316301 - Disclosure - Goodwill (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/GoodwillTables</Role>
      <ShortName>Goodwill (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cit.com/role/Goodwill</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - Business and Summary of Significant Accounting Policies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPoliciesNarrativeDetails</Role>
      <ShortName>Business and Summary of Significant Accounting Policies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BusinessAndSummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Discontinued Operations (Schedule of Condensed Balance Sheet Aerospace Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsScheduleOfCondensedBalanceSheetAerospaceDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Schedule of Condensed Balance Sheet Aerospace Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - Discontinued Operations (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsNarrativeDetails</Role>
      <ShortName>Discontinued Operations (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Discontinued Operations (Schedule of Condensed Statements of Income and Cash Flow of Aerospace Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsScheduleOfCondensedStatementsOfIncomeAndCashFlowOfAerospaceDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Schedule of Condensed Statements of Income and Cash Flow of Aerospace Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2402405 - Disclosure - Discontinued Operations (Schedule of Condensed Balance Sheet Financial Freedom Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsScheduleOfCondensedBalanceSheetFinancialFreedomDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Schedule of Condensed Balance Sheet Financial Freedom Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2402406 - Disclosure - Discontinued Operations (Schedule of Condensed Statements of Income and Cash Flow of Financial Freedom Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsScheduleOfCondensedStatementsOfIncomeAndCashFlowOfFinancialFreedomDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Schedule of Condensed Statements of Income and Cash Flow of Financial Freedom Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2402407 - Disclosure - Discontinued Operations (Schedule of Condensed Combined Balance Sheet of Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsScheduleOfCondensedCombinedBalanceSheetOfDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Schedule of Condensed Combined Balance Sheet of Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2402408 - Disclosure - Discontinued Operations (Schedule of Condensed Combined Statements of Income and Cash Flow of Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DiscontinuedOperationsScheduleOfCondensedCombinedStatementsOfIncomeAndCashFlowOfDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Schedule of Condensed Combined Statements of Income and Cash Flow of Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DiscontinuedOperationsTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Loans (Schedule of Finance Receivables by Product) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansScheduleOfFinanceReceivablesByProductDetails</Role>
      <ShortName>Loans (Schedule of Finance Receivables by Product) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2403403 - Disclosure - Loans (Schedule of Finance Receivables by Segment, Based on Obligor Location) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansScheduleOfFinanceReceivablesBySegmentBasedOnObligorLocationDetails</Role>
      <ShortName>Loans (Schedule of Finance Receivables by Segment, Based on Obligor Location) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2403404 - Disclosure - Loans (Components of Net Investment in Finance Receivables) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansComponentsOfNetInvestmentInFinanceReceivablesDetails</Role>
      <ShortName>Loans (Components of Net Investment in Finance Receivables) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2403405 - Disclosure - Loans (Finance and Held-For-Sale Receivables - By Risk Rating) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansFinanceAndHeldForSaleReceivablesByRiskRatingDetails</Role>
      <ShortName>Loans (Finance and Held-For-Sale Receivables - By Risk Rating) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2403406 - Disclosure - Loans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansNarrativeDetails</Role>
      <ShortName>Loans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2403407 - Disclosure - Loans (Schedule of Consumer Loan LTV Distributions) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansScheduleOfConsumerLoanLtvDistributionsDetails</Role>
      <ShortName>Loans (Schedule of Consumer Loan LTV Distributions) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2403408 - Disclosure - Loans (Finance and Held For Sale Receivables - Delinquency Status) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansFinanceAndHeldForSaleReceivablesDelinquencyStatusDetails</Role>
      <ShortName>Loans (Finance and Held For Sale Receivables - Delinquency Status) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2403409 - Disclosure - Loans (Finance Receivables on Non-accrual Status) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansFinanceReceivablesOnNonAccrualStatusDetails</Role>
      <ShortName>Loans (Finance Receivables on Non-accrual Status) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2403410 - Disclosure - Loans (Schedule of Loans In Process of Foreclosure) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansScheduleOfLoansInProcessOfForeclosureDetails</Role>
      <ShortName>Loans (Schedule of Loans In Process of Foreclosure) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2403411 - Disclosure - Loans (Impaired Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansImpairedLoansDetails</Role>
      <ShortName>Loans (Impaired Loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2403412 - Disclosure - Loans (Purchased Credit Impaired Loans with Deteriorated Credit Quality) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansPurchasedCreditImpairedLoansWithDeterioratedCreditQualityDetails</Role>
      <ShortName>Loans (Purchased Credit Impaired Loans with Deteriorated Credit Quality) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2403413 - Disclosure - Loans (Summary of Commercial PCI Loans by Credit Quality) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansSummaryOfCommercialPciLoansByCreditQualityDetails</Role>
      <ShortName>Loans (Summary of Commercial PCI Loans by Credit Quality) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2403414 - Disclosure - Loans (Schedule of Changes to the Accretable Yield for PCI Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansScheduleOfChangesToAccretableYieldForPciLoansDetails</Role>
      <ShortName>Loans (Schedule of Changes to the Accretable Yield for PCI Loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2403415 - Disclosure - Loans (Estimated Future Advances to Reverse Mortgages) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/LoansEstimatedFutureAdvancesToReverseMortgagesDetails</Role>
      <ShortName>Loans (Estimated Future Advances to Reverse Mortgages) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/LoansTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Allowance For Loan Losses (Schedule of Allowance for Loan Losses and Recorded Investment in Finance Receivables) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/AllowanceForLoanLossesScheduleOfAllowanceForLoanLossesAndRecordedInvestmentInFinanceReceivablesDetails</Role>
      <ShortName>Allowance For Loan Losses (Schedule of Allowance for Loan Losses and Recorded Investment in Finance Receivables) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Investment Securities (Schedule of Investment Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesScheduleOfInvestmentSecuritiesDetails</Role>
      <ShortName>Investment Securities (Schedule of Investment Securities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - Investment Securities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesNarrativeDetails</Role>
      <ShortName>Investment Securities (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - Investment Securities (Schedule of Interest and Dividend Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesScheduleOfInterestAndDividendIncomeDetails</Role>
      <ShortName>Investment Securities (Schedule of Interest and Dividend Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2405405 - Disclosure - Investment Securities (Amortized Cost and Fair Value of AFS and HTM Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesAmortizedCostAndFairValueOfAfsAndHtmSecuritiesDetails</Role>
      <ShortName>Investment Securities (Amortized Cost and Fair Value of AFS and HTM Securities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - Investment Securities (Schedule of Amortized Cost and Fair Value Maturities with Changes Recorded in Net Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesScheduleOfAmortizedCostAndFairValueMaturitiesWithChangesRecordedInNetIncomeDetails</Role>
      <ShortName>Investment Securities (Schedule of Amortized Cost and Fair Value Maturities with Changes Recorded in Net Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2405407 - Disclosure - Investment Securities (Schedule of AFS and HTM - Estimated Unrealized Losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesScheduleOfAfsAndHtmEstimatedUnrealizedLossesDetails</Role>
      <ShortName>Investment Securities (Schedule of AFS and HTM - Estimated Unrealized Losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2405408 - Disclosure - Investment Securities (Changes in Accretable Yield for Purchased Credit-Impaired Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesChangesInAccretableYieldForPurchasedCreditImpairedSecuritiesDetails</Role>
      <ShortName>Investment Securities (Changes in Accretable Yield for Purchased Credit-Impaired Securities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2405409 - Disclosure - Investment Securities (Schedule of Securities Carried at Fair Value with Changes Recorded in Net Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesScheduleOfSecuritiesCarriedAtFairValueWithChangesRecordedInNetIncomeDetails</Role>
      <ShortName>Investment Securities (Schedule of Securities Carried at Fair Value with Changes Recorded in Net Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2405410 - Disclosure - Investment Securities (Amortized Cost and Fair Value of Debt Securities Held-To-Maturity by Contractual Maturity Dates) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/InvestmentSecuritiesAmortizedCostAndFairValueOfDebtSecuritiesHeldToMaturityByContractualMaturityDatesDetails</Role>
      <ShortName>Investment Securities (Amortized Cost and Fair Value of Debt Securities Held-To-Maturity by Contractual Maturity Dates) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/InvestmentSecuritiesTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Borrowings (Schedule of Long-Term Borrowings) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BorrowingsScheduleOfLongTermBorrowingsDetails</Role>
      <ShortName>Borrowings (Schedule of Long-Term Borrowings) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BorrowingsTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Borrowings (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BorrowingsNarrativeDetails</Role>
      <ShortName>Borrowings (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BorrowingsTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2406404 - Disclosure - Borrowings (Schedule of Senior Unsecured Notes) (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.cit.com/role/BorrowingsScheduleOfSeniorUnsecuredNotesDetails</Role>
      <ShortName>Borrowings (Schedule of Senior Unsecured Notes) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BorrowingsTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2406405 - Disclosure - Borrowings (Schedule of FHLB Advances with Pledged Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BorrowingsScheduleOfFhlbAdvancesWithPledgedAssetsDetails</Role>
      <ShortName>Borrowings (Schedule of FHLB Advances with Pledged Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BorrowingsTables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2406406 - Disclosure - Borrowings (Schedule of Structured Financings and Pledged Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BorrowingsScheduleOfStructuredFinancingsAndPledgedAssetsDetails</Role>
      <ShortName>Borrowings (Schedule of Structured Financings and Pledged Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BorrowingsTables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2406407 - Disclosure - Borrowings (Assets and Liabilities in Unconsolidated VIEs) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BorrowingsAssetsAndLiabilitiesInUnconsolidatedViesDetails</Role>
      <ShortName>Borrowings (Assets and Liabilities in Unconsolidated VIEs) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BorrowingsTables</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Derivative Financial Instruments (Fair and Notional Values of Derivative Financial Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsFairAndNotionalValuesOfDerivativeFinancialInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments (Fair and Notional Values of Derivative Financial Instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Derivative Financial Instruments (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsNarrativeDetails</Role>
      <ShortName>Derivative Financial Instruments (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2407404 - Disclosure - Derivative Financial Instruments (Offsetting of Derivative Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsOffsettingOfDerivativeAssetsDetails</Role>
      <ShortName>Derivative Financial Instruments (Offsetting of Derivative Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2407405 - Disclosure - Derivative Financial Instruments (Offsetting of Derivative Liabilities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsOffsettingOfDerivativeLiabilitiesDetails</Role>
      <ShortName>Derivative Financial Instruments (Offsetting of Derivative Liabilities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2407406 - Disclosure - Derivative Financial Instruments (Derivative Instrument Gains and Losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsDerivativeInstrumentGainsAndLossesDetails</Role>
      <ShortName>Derivative Financial Instruments (Derivative Instrument Gains and Losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2407407 - Disclosure - Derivative Financial Instruments (Changes in AOCI Relating to Derivatives) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/DerivativeFinancialInstrumentsChangesInAociRelatingToDerivativesDetails</Role>
      <ShortName>Derivative Financial Instruments (Changes in AOCI Relating to Derivatives) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/DerivativeFinancialInstrumentsTables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Fair Value (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Fair Value (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValueTables</ParentRole>
      <Position>83</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Fair Value (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueNarrativeDetails</Role>
      <ShortName>Fair Value (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValueTables</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - Fair Value (Quantitative Information about Level 3 Fair Value Measurements-Recurring) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueQuantitativeInformationAboutLevel3FairValueMeasurementsRecurringDetails</Role>
      <ShortName>Fair Value (Quantitative Information about Level 3 Fair Value Measurements-Recurring) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValueTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2408405 - Disclosure - Fair Value (Changes in Estimated Fair Value for Financial Assets and Liabilities Measured on Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueChangesInEstimatedFairValueForFinancialAssetsAndLiabilitiesMeasuredOnRecurringBasisDetails</Role>
      <ShortName>Fair Value (Changes in Estimated Fair Value for Financial Assets and Liabilities Measured on Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValueTables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2408406 - Disclosure - Fair Value (Carrying Value of Assets Measured at Fair Value on a Non-Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueCarryingValueOfAssetsMeasuredAtFairValueOnNonRecurringBasisDetails</Role>
      <ShortName>Fair Value (Carrying Value of Assets Measured at Fair Value on a Non-Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValueTables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - Fair Value (Carrying and Estimated Fair Values of Financial Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/FairValueCarryingAndEstimatedFairValuesOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value (Carrying and Estimated Fair Values of Financial Instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/FairValueTables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Stockholder's Equity (Schedule of Common Stock Activity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquityScheduleOfCommonStockActivityDetails</Role>
      <ShortName>Stockholder's Equity (Schedule of Common Stock Activity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - Stockholders' Equity (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquityNarrativeDetails</Role>
      <ShortName>Stockholders' Equity (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/StockholdersEquityTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2409404 - Disclosure - Stockholders' Equity (Components of Accumulated Other Comprehensive Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquityComponentsOfAccumulatedOtherComprehensiveLossDetails</Role>
      <ShortName>Stockholders' Equity (Components of Accumulated Other Comprehensive Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/StockholdersEquityTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2409405 - Disclosure - Stockholders' Equity (Changes in Accumulated Other Comprehensive Income (Loss) by Component) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquityChangesInAccumulatedOtherComprehensiveIncomeLossByComponentDetails</Role>
      <ShortName>Stockholders' Equity (Changes in Accumulated Other Comprehensive Income (Loss) by Component) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/StockholdersEquityTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2409406 - Disclosure - Stockholders' Equity (Reclassifications out of Accumulated Other Comprehensive (Income) Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/StockholdersEquityReclassificationsOutOfAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Stockholders' Equity (Reclassifications out of Accumulated Other Comprehensive (Income) Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/StockholdersEquityTables</ParentRole>
      <Position>93</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Regulatory Capital (Tier 1 Capital and Total Capital Components) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/RegulatoryCapitalTier1CapitalAndTotalCapitalComponentsDetails</Role>
      <ShortName>Regulatory Capital (Tier 1 Capital and Total Capital Components) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/RegulatoryCapitalTables</ParentRole>
      <Position>94</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2411401 - Disclosure - Income Taxes (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/IncomeTaxesNarrativeDetails</Role>
      <ShortName>Income Taxes (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/IncomeTaxes</ParentRole>
      <Position>95</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Commitments (Summary of Commitments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/CommitmentsSummaryOfCommitmentsDetails</Role>
      <ShortName>Commitments (Summary of Commitments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/CommitmentsTables</ParentRole>
      <Position>96</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2412403 - Disclosure - Commitments (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/CommitmentsNarrativeDetails</Role>
      <ShortName>Commitments (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/CommitmentsTables</ParentRole>
      <Position>97</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2413401 - Disclosure - Contingencies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/ContingenciesNarrativeDetails</Role>
      <ShortName>Contingencies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/Contingencies</ParentRole>
      <Position>98</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2414401 - Disclosure - Certain Relationships and Related Transactions (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/CertainRelationshipsAndRelatedTransactionsNarrativeDetails</Role>
      <ShortName>Certain Relationships and Related Transactions (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/CertainRelationshipsAndRelatedTransactions</ParentRole>
      <Position>99</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Business Segment Information (Segment Pre-Tax Income (Loss)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/BusinessSegmentInformationSegmentPreTaxIncomeLossDetails</Role>
      <ShortName>Business Segment Information (Segment Pre-Tax Income (Loss)) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/BusinessSegmentInformationTables</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2416402 - Disclosure - Goodwill (Schedule of Goodwill) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/GoodwillScheduleOfGoodwillDetails</Role>
      <ShortName>Goodwill (Schedule of Goodwill) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/GoodwillTables</ParentRole>
      <Position>101</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2416403 - Disclosure - Goodwill (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/GoodwillNarrativeDetails</Role>
      <ShortName>Goodwill (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/GoodwillTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report instance="cit-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2417401 - Disclosure - Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cit.com/role/SubsequentEventsDetails</Role>
      <ShortName>Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cit.com/role/SubsequentEvents</ParentRole>
      <Position>103</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>cit-20170930.xml</File>
    <File>cit-20170930.xsd</File>
    <File>cit-20170930_cal.xml</File>
    <File>cit-20170930_def.xml</File>
    <File>cit-20170930_lab.xml</File>
    <File>cit-20170930_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2014-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
