Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Investments [Line Items]
Securities pledged as collateral	$ 1,800,000	$ 0
Amortized Cost	2,655,314	2,575,719
Gross Unrealized Gains	8,026	13,062
Gross Unrealized Losses	(70,517)	(40,825)
Estimated Fair Value	2,592,823	2,547,956
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	21,105	5,962
Gross Unrealized Gains	40	2
Gross Unrealized Losses	(183)	(26)
Estimated Fair Value	20,962	5,938
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	413,632	405,860
Gross Unrealized Gains	2,247	5,638
Gross Unrealized Losses	(9,780)	(2,549)
Estimated Fair Value	406,099	408,949
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	101,367	96,353
Gross Unrealized Gains	2,482	2,832
Gross Unrealized Losses	(1,894)	(1,876)
Estimated Fair Value	101,955	97,309
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	695,840	611,927
Gross Unrealized Gains	236	491
Gross Unrealized Losses	(16,376)	(9,795)
Estimated Fair Value	679,700	602,623
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,084,582	1,132,080
Gross Unrealized Gains	3,006	3,957
Gross Unrealized Losses	(33,422)	(15,241)
Estimated Fair Value	1,054,166	1,120,796
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	231,378	215,351
Gross Unrealized Gains	15	0
Gross Unrealized Losses	(4,501)	(2,596)
Estimated Fair Value	226,892	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	(4,361)	(8,742)
Estimated Fair Value	103,049	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