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Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. 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This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. false2Cash and cash equivalents, Carrying Amount4false 5us-gaap_HeldToMaturitySecuritiesFairValueus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse891000000891USD$falsefalsefalse2truefalsefalse11350000001135USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThis item represents the aggregate fair value of investments in debt securities as of the balance sheet date and which are categorized as held-to-maturity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 19 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. 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This item represents the amount of short-term debt existing as of the balance sheet date.No definition available.false2Commercial paper and other short-term borrowings, Fair Value16false 5us-gaap_ShortTermBorrowingsus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse37920000003792USD$falsefalsefalse2truefalsefalse13630000001363USD$falsefalsefalsexbrli:monetaryItemTypemonetaryReflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph a -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),16(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Subparagraph a(1) -Article 7 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2, 3 -Article 9 false2Commercial paper and other short-term borrowings, Carrying Amount17false 5us-gaap_LongTermDebtFairValueus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse61840000006184USD$falsefalsefalse2truefalsefalse58610000005861USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. 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Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27232-111563 false2Securities held to maturity, Fair Value28true 4us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Liabilities29false 5us-gaap_DepositsFairValueDisclosureus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse7751200000077512USD$falsefalsefalse2truefalsefalse7452400000074524USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThis element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure. This item represents the aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits among others.No definition available.false2Deposits, Fair Value30false 5us-gaap_ShorttermDebtFairValueus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse37920000003792USD$falsefalsefalse2truefalsefalse13630000001363USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThis element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents the amount of short-term debt existing as of the balance sheet date.No definition available.false2Commercial paper and other short-term borrowings, Fair Value31false 5us-gaap_LongTermDebtFairValueus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse61840000006184USD$falsefalsefalse2truefalsefalse58610000005861USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.No definition available.false2Long-term debt, Fair Value32false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse7false USDtruefalse$I2013Q2_FairValueInputsLevel3Memberhttp://www.sec.gov/CIK0001011659instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseLevel 3 [Member]us-gaap_FairValueByFairValueHierarchyLevelAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_FairValueInputsLevel3Memberus-gaap_FairValueByFairValueHierarchyLevelAxisexplicitMemberUSDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Level 3 [Member]33true 4us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Assets34false 5ub_LoansHeldForInvestmentNetOfAllowanceForLoanLossesAtFairValueub_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse6596400000065964USD$falsefalsefalse2truefalsefalse5961300000059613USD$falsefalsefalsexbrli:monetaryItemTypemonetaryLoans held for investment, net of allowance for loan losses at fair value.No definition available.false2Loans held for investment, net of allowance for loan losses, Fair Value35false 5ub_FdicIndemnificationAssetAtFairValueub_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse131000000131USD$falsefalsefalse2truefalsefalse151000000151USD$falsefalsefalsexbrli:monetaryItemTypemonetaryFDIC indemnification asset at fair value.No definition available.false2FDIC indemnification asset, Fair Value36false 5us-gaap_OtherAssetsFairValueDisclosureus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse20000002USD$falsefalsefalse2truefalsefalse30000003USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThis element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of assets categorized as other which are not in and of themselves material enough to require separate disclosure.No definition available.false2Other assets, Fair Value37true 4ub_OffBalanceSheetInstrumentsAbstractub_falsenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Off-Balance Sheet Instruments38false 5ub_CommitmentsToExtendCreditAndStandbyAndCommercialLettersOfCreditFairValueDisclosureub_falsecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse277000000277USD$falsetruefalse2truefalsefalse262000000262USD$falsetruefalsexbrli:monetaryItemTypemonetaryCommitments to extend credit and standby and commercial letters of credit, fair value disclosure.No definition available.false2Commitments to extend credit and standby and commercial letters of credit, Fair ValuefalseFair Value Measurement and Fair Value of Financial Instruments - Carrying Amount and Estimated Fair Value of Certain Financial Instruments (Detail) (USD $)MillionsUnKnownUnKnownUnKnowntruefalsefalseSheethttp://www.unionbank.com/role/DisclosureFairValueMeasurementAndFairValueOfFinancialInstrumentsCarryingAmountAndEstimatedFairValueOfCertainFinancialInstruments238