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Securities Fair Value Option Securities (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Schedule of Fair Value Option Securities [Line Items]      
Fair Value Option Securities, Fair Value $ 434,077bokf_FairValueOptionSecurities $ 311,597bokf_FairValueOptionSecurities $ 160,884bokf_FairValueOptionSecurities
Fair Value Option Securities, Unrealized Gain or Loss 4,271bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss 1,624bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss (5,510)bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
U.S. agency residential mortgage-backed securities [Member]      
Schedule of Fair Value Option Securities [Line Items]      
Fair Value Option Securities, Fair Value 434,077bokf_FairValueOptionSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 311,597bokf_FairValueOptionSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 156,525bokf_FairValueOptionSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fair Value Option Securities, Unrealized Gain or Loss 4,271bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 1,624bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 (5,794)bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Other debt securities [Member]      
Schedule of Fair Value Option Securities [Line Items]      
Fair Value Option Securities, Fair Value 0bokf_FairValueOptionSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		 0bokf_FairValueOptionSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		 4,359bokf_FairValueOptionSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember
Fair Value Option Securities, Unrealized Gain or Loss $ 0bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		 $ 0bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember		 $ 284bokf_FairValueOptionSecuritiesUnrealizedGainOrLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherDebtSecuritiesMember