Related Parties Related Parties (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Directors and Senior Management [Member]
Loans and Leases Receivable, Related Parties [Roll Forward]
Beginning balance		$ 75,189	$ 75,265
Advances		498,425	886,610
Payments		(484,958)	(896,643)
Adjustments	[1]	4,284	9,957
Ending balance		92,940	75,189
Director [Member]
Loan Commitments and Equity Investments [Abstract]
Related Party Transaction, Expenses from Transactions with Related Party		$ 10,000
Executive Officer [Member]
Loan Commitments and Equity Investments [Abstract]
Percentage of investment funds' assets held by clients		80.00%
Assets Held in Cavanal Hill Funds		$ 3,700,000
Subsidiaries [Member] | BOKF, NA [Member] | Affiliated Entity [Member]
Loan Commitments and Equity Investments [Abstract]
Maximum percentage of unimpaired capital on loan commitments and equity investments to a single affiliate (in hundredths)		10.00%
Maximum percentage of unimpaired capital on loan commitments and equity investments to all affiliates (in hundredths)		20.00%
Maximum loan commitments and equity investments to a single affiliate		$ 416,000
Maximum loan commitments and equity investments to all affiliates		832,000
Largest loan commitment and equity investment to a single affiliate		264,000
Aggregate loan commitment and equity investment to all affiliates		324,000	392,000
Outstanding amounts to all affiliates		5,300	$ 5,000
Largest amount outstanding to a single affiliate		$ 4,100

[1] Adjustments generally consist of changes in status as a related party.