Loans Loans, Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	$ 278,335	$ 274,264
Charge offs	(56,246)	(86,439)
Recoveries	16,791	21,510
Net charge offs	(39,455)	(64,929)
Provision for Loan Losses	27,000	69,000
Balance at end of period	265,880	278,335
Allowance for loan losses
Individually evaluated	25,846	35,054
Collectively evaluated	240,034	243,281
Balance at end of period	265,880	278,335
Loans
Individually evaluated	247,575	316,735
Collectively evaluated	20,537,416	19,737,981
Loans and Leases Receivable, Net of Deferred Income	20,784,991	20,054,716
Oil and Gas Portfolio Segment [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of year	38,000	42,000
Charge offs	(25,000)	(59,000)
Recoveries	0	0
Net charge offs	(25,000)	(59,000)
Provision for Loan Losses	14,000	55,000
Balance at end of period	27,000	38,000
Allowance for loan losses
Individually evaluated	5,000	14,000
Collectively evaluated	22,000	24,000
Balance at end of period	27,000	38,000
Loans
Individually evaluated	77,000	147,000
Collectively evaluated	523,000	521,000
Loans and Leases Receivable, Net of Deferred Income	$ 600,000	$ 668,000