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, 2018	Dec. 31, 2017	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 265,880	$ 278,335
Charge offs	(44,385)	(56,246)
Recoveries	14,255	16,791
Net Charge offs	(30,130)	(39,455)
Provision for loan losses	500	27,000
Balance at end of period	236,250	265,880
Allowance for loan losses
Individually evaluated			$ 15,781	$ 25,846
Collectively evaluated			220,469	240,034
Total allowance for loan losses	265,880	265,880	236,250	265,880
Loans
Individually evaluated			168,568	247,575
Collectively evaluated			22,685,411	20,537,416
Total loans			22,867,112	20,784,991
Oil and Gas Portfolio Segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	27,000	38,000
Charge offs	(24,000)	(25,000)
Recoveries	3,000	0
Net Charge offs	(20,000)	(25,000)
Provision for loan losses	3,000	14,000
Balance at end of period	10,000	27,000
Allowance for loan losses
Individually evaluated			0	5,000
Collectively evaluated			10,000	22,000
Total allowance for loan losses	$ 10,000	$ 38,000	10,000	27,000
Loans
Individually evaluated			32,000	77,000
Collectively evaluated			700,000	523,000
Total loans			$ 731,000	$ 600,000