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Commitments, Guarantees, And Contingent Liabilities (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Guarantees, Commitments And Contingencies [Abstract]  
Letters of credit, accrued liability $ 15.0zion_LettersOfCreditAccruedLiabilityForGuarantees
Reserve for unfunded lending commitments 12.0zion_LiabilityForGuaranteeReserveForUnfundedLendingCommitments
Deferred commitment fees 3.0zion_LiabilityForGuaranteesDeferedCommitmentFees
Guarantee of trust preferred securities issued by affiliated trusts 15.0zion_GuaranteeOfTrustPreferredSecuritiesIssuedByAffiliatedTrusts
Commitments for Private Equity and Other Noninterest-bearing Investments 34.9zion_CommitmentsForPrivateEquityAndOtherNoninterestBearingInvestments
Range of possible loss, minimum 0us-gaap_LossContingencyRangeOfPossibleLossMinimum
Range of possible loss, maximum $ 50us-gaap_LossContingencyRangeOfPossibleLossMaximum