Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Investments [Line Items]
Securities pledged as collateral	$ 1,900,000	$ 1,800,000
Amortized Cost	2,576,201	2,594,255
Gross Unrealized Gains	20,665	24,807
Gross Unrealized Losses	(35,350)	(59,835)
Estimated Fair Value	2,561,516	2,559,227
Other Equity Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	1,000	1,000
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	5,961	132
Gross Unrealized Gains	54	2
Gross Unrealized Losses	0	0
Estimated Fair Value	6,015	134
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	415,313	405,274
Gross Unrealized Gains	4,005	2,043
Gross Unrealized Losses	(5,405)	(15,676)
Estimated Fair Value	413,913	391,641
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	92,355	112,016
Gross Unrealized Gains	2,578	1,978
Gross Unrealized Losses	(1,956)	(4,585)
Estimated Fair Value	92,977	109,409
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	601,845	604,095
Gross Unrealized Gains	1,380	1,943
Gross Unrealized Losses	(9,547)	(12,178)
Estimated Fair Value	593,678	593,860
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,184,797	1,328,192
Gross Unrealized Gains	5,850	6,546
Gross Unrealized Losses	(8,561)	(16,900)
Estimated Fair Value	1,182,086	1,317,838
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	161,960	25,100
Gross Unrealized Gains	299	0
Gross Unrealized Losses	(627)	(537)
Estimated Fair Value	161,632	24,563
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	107,410	107,215
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9,254)	(9,959)
Estimated Fair Value	98,156	97,256
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	2,569,641	2,582,024
Gross Unrealized Gains	14,166	12,512
Gross Unrealized Losses	(35,350)	(59,835)
Estimated Fair Value	2,548,457	2,534,701
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	6,560	12,231
Gross Unrealized Gains	6,499	12,295
Gross Unrealized Losses	0	0
Estimated Fair Value	13,059	$ 24,526
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	$ 10,000		$ 10,000