Loans, Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	$ 265,880	$ 278,335
Charge offs	(50,536)	(56,246)
Recoveries	20,179	16,791
Net charge offs	(30,358)	(39,455)
Provision for Loan Losses	2,500	27,000
Balance at end of period	238,023	265,880
Allowance for loan losses
Individually evaluated	15,304	25,846
Collectively evaluated	222,719	240,034
Balance at end of period	238,023	265,880
Loans
Individually evaluated	133,690	247,575
Collectively evaluated	22,801,887	20,537,416
Loans and Leases Receivable, Net of Deferred Income	22,940,429	20,784,991
Oil and Gas Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	27,000	38,000
Charge offs	(24,000)	(25,000)
Recoveries	6,000	0
Net charge offs	(17,000)	(25,000)
Provision for Loan Losses	2,000	14,000
Balance at end of period	12,000	27,000
Allowance for loan losses
Individually evaluated	0	5,000
Collectively evaluated	12,000	22,000
Balance at end of period	12,000	27,000
Loans
Individually evaluated	22,000	77,000
Collectively evaluated	725,000	523,000
Loans and Leases Receivable, Net of Deferred Income	$ 747,000	$ 600,000