Loans, Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	$ 238,023	$ 265,880
Charge offs	(74,313)	(50,536)
Recoveries	19,161	20,179
Net charge offs	(55,152)	(30,358)
Provision for Loan Losses	18,500	2,500
Balance at end of period	201,371	238,023
Allowance for loan losses
Individually evaluated	16,165	15,304
Collectively evaluated	185,205	222,719
Balance at end of period	201,371	238,023
Loans
Individually evaluated	110,043	133,690
Collectively evaluated	22,709,845	22,801,887
Loans and Leases Receivable, Net of Deferred Income	22,821,440	22,940,429
Oil and Gas Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	12,000	27,000
Charge offs	(50,000)	(24,000)
Recoveries	5,000	6,000
Net charge offs	(44,000)	(17,000)
Provision for Loan Losses	45,000	2,000
Balance at end of period	12,000	12,000
Allowance for loan losses
Individually evaluated	3,000	0
Collectively evaluated	9,000	12,000
Balance at end of period	$ 12,000	$ 12,000
Percent of Loans	2.56%	1.62%
Loans
Individually evaluated	$ 23,000	$ 22,000
Collectively evaluated	460,000	725,000
Loans and Leases Receivable, Net of Deferred Income	$ 484,000	$ 747,000