Loans Loans, Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 278,335	$ 274,264
Charge offs	(41,957)	(86,439)
Recoveries	13,173	21,510
Net Charge offs	(28,784)	(64,929)
Provision for loan losses	27,000	69,000
Balance at end of period	276,551	278,335
Allowance for loan losses
Individually evaluated			$ 25,671	$ 35,054
Collectively evaluated			250,880	243,281
Total allowance for loan losses	276,551	278,335	276,551	278,335
Loans
Individually evaluated			252,542	316,735
Collectively evaluated			20,678,918	19,737,981
Total loans			20,931,460	20,054,716
Oil and Gas Portfolio Segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	38,000	42,000
Charge offs	(26,000)	(59,000)
Recoveries	0	0
Net Charge offs	(26,000)	(59,000)
Provision for loan losses	18,000	55,000
Balance at end of period	30,000	38,000
Allowance for loan losses
Individually evaluated			2,000	14,000
Collectively evaluated			28,000	24,000
Total allowance for loan losses	$ 30,000	$ 42,000	30,000	38,000
Loans
Individually evaluated			92,000	147,000
Collectively evaluated			485,000	521,000
Total loans			$ 577,000	$ 668,000