Loans - Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 265,880	$ 278,335
Charge offs	(12,155)	(56,246)
Recoveries	2,832	16,791
Net Charge offs	(9,323)	(39,455)
Provision for loan losses	500	27,000
Balance at end of period	257,058	265,880
Allowance for loan losses
Individually evaluated			$ 34,094	$ 25,846
Collectively evaluated			222,964	240,034
Total allowance for loan losses	265,880	278,335	257,058	265,880
Loans
Individually evaluated			233,078	247,575
Collectively evaluated			22,562,575	20,537,416
Total loans			22,810,491	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	(4,000)	(25,000)
Recoveries	0	0
Net Charge offs	(4,000)	(25,000)
Provision for loan losses	(4,000)	14,000
Balance at end of period	19,000	27,000
Allowance for loan losses
Individually evaluated			10,000	5,000
Collectively evaluated			9,000	22,000
Total allowance for loan losses	$ 27,000	$ 38,000	19,000	27,000
Loans
Individually evaluated			69,000	77,000
Collectively evaluated			588,000	523,000
Total loans			$ 657,000	$ 600,000