Loans Loans Changes in the Allowance for Loan Losses by Oil and Gas Segment (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period		$ 238,023	$ 265,880
Charge offs		(31,247)	(50,536)
Recoveries		10,384	20,179
Net Charge offs		(20,864)	(30,358)
Provision for loan losses		16,500	2,500
Balance at end of period		233,659	238,023
Allowance for loan losses
Individually evaluated for impairment				$ 32,188	$ 15,304
Collectively evaluated for impairment				201,470	222,719
Total allowance for loan losses		238,023	238,023	233,659	238,023
Loans
Individually evaluated for impairment				172,489	133,690
Collectively evaluated for impairment				23,075,334	22,801,887
Total loans	[1]			23,249,967	22,940,429
Oil and Gas
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Balance at beginning of period		12,000	27,000
Charge offs		(17,000)	(24,000)
Recoveries		4,000	6,000
Net Charge offs		(13,000)	(17,000)
Provision for loan losses		26,000	2,000
Balance at end of period		25,000	12,000
Allowance for loan losses
Individually evaluated for impairment				16,000	0
Collectively evaluated for impairment				9,000	12,000
Total allowance for loan losses		$ 25,000	$ 27,000	25,000	12,000
Loans
Individually evaluated for impairment				63,000	22,000
Collectively evaluated for impairment				594,000	725,000
Total loans				$ 657,000	$ 747,000

[1] (a) Includes $2 million and $5 million of purchased credit-impaired loans at June 30, 2019 and December 31, 2018, respectively.