Fair Value (Narrative) (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) holdback		Dec. 31, 2016 USD ($)		Sep. 30, 2016 USD ($)
Fair Value Disclosure [Line Items]
Borrower rate, max effective period	90 days
Assets held for sale	$ 2,162,000,000	[1]	$ 636,000,000	[1]	$ 1,406,700,000
Fair value of loans, percentage	101.60%		100.60%
Unpaid principal balance	$ 3,265,800,000		$ 3,685,000,000
Impaired loans carrying amount	$ 210,600,000		$ 188,200,000
Carrying amount of impaired loans percentage of unpaid principal balance	80.50%		77.00%
Impaired loans unpaid principal balance with no specific allowance	$ 74,300,000		$ 74,700,000
Impaired loans carrying value with no specific allowance	61,700,000		55,500,000
Impaired Loans
Fair Value Disclosure [Line Items]
Unpaid principal balance	261,500,000		244,300,000
Level 2
Fair Value Disclosure [Line Items]
Assets held for sale	3,500,000
Unsecured borrowings	3,800,000,000		10,600,000,000
Face amount	3,800,000,000		3,300,000,000
Level 3
Fair Value Disclosure [Line Items]
Face amount	1,000,000,000		1,100,000,000
Estimated Fair Value
Fair Value Disclosure [Line Items]
Fair value of loans	26,178,600,000		26,846,700,000
Estimated Fair Value | Level 2
Fair Value Disclosure [Line Items]
Fair value of loans	459,900,000		390,300,000
Estimated Fair Value | Level 3
Fair Value Disclosure [Line Items]
OREO estimated fair value	27,300,000
Fair value of loans	25,718,700,000		$ 26,456,400,000
Minimum
Fair Value Disclosure [Line Items]
Threshold at which impaired finance receivables that are placed on non-accrual status are subject to individual review	$ 500,000
Minimum | Other Real Estate Owned | Level 3
Fair Value Disclosure [Line Items]
Inputs used to estimate cost to sell	5.30%
Maximum | Other Real Estate Owned | Level 3
Fair Value Disclosure [Line Items]
Inputs used to estimate cost to sell	7.80%
Weighted Average | Other Real Estate Owned | Level 3
Fair Value Disclosure [Line Items]
Inputs used to estimate cost to sell	5.70%
Onewest Bank
Fair Value Disclosure [Line Items]
Probable amount of holdback to be paid	$ 62,400,000
Contingent liabilities	46,000,000
Onewest Bank | Minimum
Fair Value Disclosure [Line Items]
Range of potential holdback to be paid	0
Onewest Bank | Maximum
Fair Value Disclosure [Line Items]
Range of potential holdback to be paid	$ 116,000,000
FDIC True-up liability | La Jolla Transaction
Fair Value Disclosure [Line Items]
Number of days after the loss sharing agreement maturity	45 days
Consideration holdback liability | Onewest Bank
Fair Value Disclosure [Line Items]
Number of consideration holdback liabilities | holdback	4
Reduction in cash consideration due to trailing risks	$ 116,000,000
Consideration holdback liability | Onewest Bank | Minimum
Fair Value Disclosure [Line Items]
Holdback periods	1 year
Consideration holdback liability | Onewest Bank | Maximum
Fair Value Disclosure [Line Items]
Holdback periods	5 years

[1] The following table presents information on assets and liabilities related to Variable Interest Entities (VIEs) that are consolidated by the Company. The difference between VIE total assets and total liabilities represents the Company’s interests in those entities, which were eliminated in consolidation. The assets of the consolidated VIEs will be used to settle the liabilities of those entities and, except for the Company’s interest in the VIEs, are not available to the creditors of CIT or any affiliates of CIT.Assets Cash and interest bearing deposits, restricted$88.3 $99.9Total loans, net of allowance for loan losses146.8 300.5Operating lease equipment, net759.9 775.8Assets of discontinued operations— 2,321.7Total Assets$995.0 $3,497.9Liabilities Beneficial interests issued by consolidated VIEs (classified as long-term borrowings)$603.9 $770.0Liabilities of discontinued operations— 1,204.6Total Liabilities$603.9 $1,974.6