Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Investments [Line Items]
Securities pledged as collateral	$ 1,500,000	$ 1,800,000
Amortized Cost	2,664,163	2,575,719
Gross Unrealized Gains	6,149	13,062
Gross Unrealized Losses	(77,029)	(40,825)
Estimated Fair Value	2,593,283	2,547,956
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	26,830	5,962
Gross Unrealized Gains	5	2
Gross Unrealized Losses	(309)	(26)
Estimated Fair Value	26,526	5,938
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	415,460	405,860
Gross Unrealized Gains	2,101	5,638
Gross Unrealized Losses	(9,865)	(2,549)
Estimated Fair Value	407,696	408,949
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	92,411	96,353
Gross Unrealized Gains	1,261	2,832
Gross Unrealized Losses	(1,802)	(1,876)
Estimated Fair Value	91,870	97,309
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	742,282	611,927
Gross Unrealized Gains	406	491
Gross Unrealized Losses	(18,366)	(9,795)
Estimated Fair Value	724,322	602,623
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,035,724	1,132,080
Gross Unrealized Gains	2,342	3,957
Gross Unrealized Losses	(37,102)	(15,241)
Estimated Fair Value	1,000,964	1,120,796
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	244,046	215,351
Gross Unrealized Gains	34	0
Gross Unrealized Losses	(5,297)	(2,596)
Estimated Fair Value	238,783	212,755
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	107,410	107,410
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(4,288)	(8,742)
Estimated Fair Value	103,122	98,668
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost		2,574,943
Gross Unrealized Gains		12,920
Gross Unrealized Losses		(40,825)
Estimated Fair Value		2,547,038
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost		776
Gross Unrealized Gains		142
Gross Unrealized Losses		0
Estimated Fair Value		$ 918
Pooled Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 11,500		$ 11,500