Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Amortized Cost	$ 2,494,087	$ 2,494,059
Gross Unrealized Gains	53,050	28,740
Gross Unrealized Losses	(17,413)	(38,026)
Estimated Fair Value	2,529,724	2,484,773
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	143	25,154
Gross Unrealized Gains	3	35
Gross Unrealized Losses	0	(53)
Estimated Fair Value	146	25,136
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	333,246	256,746
Gross Unrealized Gains	12,101	6,019
Gross Unrealized Losses	0	0
Estimated Fair Value	345,347	262,765
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	95,419	100,336
Gross Unrealized Gains	3,001	2,695
Gross Unrealized Losses	(6,873)	(6,076)
Estimated Fair Value	91,547	96,955
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	701,853	835,439
Gross Unrealized Gains	5,951	3,042
Gross Unrealized Losses	(1,458)	(16,972)
Estimated Fair Value	706,346	821,509
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,242,267	1,154,935
Gross Unrealized Gains	25,501	10,104
Gross Unrealized Losses	(5)	(6,204)
Estimated Fair Value	1,267,763	1,158,835
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	106,949	106,772
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9,063)	(8,713)
Estimated Fair Value	97,886	98,059
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	2,479,877	2,479,382
Gross Unrealized Gains	46,557	21,895
Gross Unrealized Losses	(17,399)	(38,018)
Estimated Fair Value	2,509,035	2,463,259
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	14,210	14,677
Gross Unrealized Gains	6,493	6,845
Gross Unrealized Losses	(14)	(8)
Estimated Fair Value	$ 20,689	$ 21,514