Securities Fair Value Option Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of Fair Value Option Securities [Line Items]
Fair Value Option Securities, Fair Value	$ 452,150	$ 755,054	$ 819,531
U.S. government agency residential mortgage-backed securities [Member]
Schedule of Fair Value Option Securities [Line Items]
Fair Value Option Securities, Fair Value	452,150	755,054	819,531
Fair Value Option Securities, Net Unrealized Gain or Loss	$ (7,923)	$ (1,877)	$ 1,671

Fair value option securities represent securities the Company has elected to carry at fair value and separately identified on the balance sheet with changes in fair value recognized in earnings as they occur. Certain residential mortgage-backed securities issued by US government agencies are held as an economic hedge of changes in the fair value of mortgage servicing rights and related derivative contracts. In addition, other securities may be held to manage interest rate risk or for deferred compensation purposes. Securities held for interest rate risk management may be economically hedged by derivative contracts not designated as hedging instruments effectively modify these fixed rate securities into variable rate securities.

No definition available.

Name:	bokf_FairValueOptionSecurities
Namespace Prefix:	bokf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant