Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Schedule of Investments [Line Items]
Securities pledged as collateral	$ 1,700,000	$ 1,800,000
Amortized Cost	2,503,895	2,594,255
Gross Unrealized Gains	24,890	24,807
Gross Unrealized Losses	(40,086)	(59,835)
Estimated Fair Value	2,488,699	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	5,960	132
Gross Unrealized Gains	94	2
Gross Unrealized Losses	0	0
Estimated Fair Value	6,054	134
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	405,035	405,274
Gross Unrealized Gains	3,726	2,043
Gross Unrealized Losses	(7,125)	(15,676)
Estimated Fair Value	401,636	391,641
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	88,887	112,016
Gross Unrealized Gains	2,531	1,978
Gross Unrealized Losses	(2,426)	(4,585)
Estimated Fair Value	88,992	109,409
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	546,847	604,095
Gross Unrealized Gains	1,399	1,943
Gross Unrealized Losses	(9,739)	(12,178)
Estimated Fair Value	538,507	593,860
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,218,257	1,328,192
Gross Unrealized Gains	5,782	6,546
Gross Unrealized Losses	(10,974)	(16,900)
Estimated Fair Value	1,213,065	1,317,838
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	121,012	25,100
Gross Unrealized Gains	390	0
Gross Unrealized Losses	(335)	(537)
Estimated Fair Value	121,067	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	(9,487)	(9,959)
Estimated Fair Value	97,923	97,256
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	2,493,408	2,582,024
Gross Unrealized Gains	13,922	12,512
Gross Unrealized Losses	(40,086)	(59,835)
Estimated Fair Value	2,467,244	2,534,701
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	10,487	12,231
Gross Unrealized Gains	10,968	12,295
Gross Unrealized Losses	0	0
Estimated Fair Value	21,455	$ 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