2.4.0.84320 - Disclosure - Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Detail)truefalseIn Millions, unless otherwise specifiedfalse1false USDfalsefalse$I2013Q2http://www.sec.gov/CIK0001011659instant2013-06-30T00:00:000001-01-01T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$2false USDfalsefalse$I2012http://www.sec.gov/CIK0001011659instant2012-12-31T00:00:000001-01-01T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$1true 1us-gaap_ShortTermBorrowingsAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Short Term Borrowings [Abstract]2false 2us-gaap_FederalFundsPurchasedAndSecuritiesSoldUnderAgreementsToRepurchaseus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse137000000137USD$falsetruefalse2truefalsefalse123000000123USD$falsetruefalsexbrli:monetaryItemTypemonetaryThe amount shown on the books that a bank with insufficient reserves borrows, at the federal funds rate, from another bank to meet its reserve requirements and the amount of securities that an institution sells and agrees to repurchase at a specified date for a specified price, net of any reductions or offsets.Reference 1: 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(1)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 1 -Article 9 false2Federal funds purchased and securities sold under repurchase agreements with weighted average interest rates of 0.07% and 0.96% at June 30, 2013 and December 31, 2012, respectively3false 2us-gaap_CommercialPaperus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse36550000003655falsefalsefalse2truefalsefalse12400000001240falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: 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)(3)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 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(2)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph a(3) -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2 -Article 9 false2Commercial paper, with weighted average interest rates of 0.17% and 0.20% at June 30, 2013 and December 31, 2012, respectively4true 2ub_OtherBorrowedFundsAbstractub_falsenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Other borrowed funds:5false 3us-gaap_ShortTermBorrowingsus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalsetotalLabel1truefalsefalse37920000003792USD$falsetruefalse2truefalsefalse13630000001363USD$falsetruefalsexbrli: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 true2Total commercial paper and other short-term borrowingsfalseCommercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Detail) (USD $)MillionsUnKnownUnKnownUnKnowntruefalsefalseSheethttp://www.unionbank.com/role/DisclosureCommercialPaperAndOtherShortTermBorrowingsSummaryOfCommercialPaperAndOtherShortTermBorrowings25