Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Amortized Cost	$ 2,500,245	$ 2,494,059
Gross Unrealized Gains	39,450	28,740
Gross Unrealized Losses	(23,490)	(38,026)
Estimated Fair Value	2,516,205	2,484,773
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	14,228	14,677
Gross Unrealized Gains	4,852	6,845
Gross Unrealized Losses	(13)	(8)
Estimated Fair Value	19,067	21,514
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	25,149	25,154
Gross Unrealized Gains	24	35
Gross Unrealized Losses	0	(53)
Estimated Fair Value	25,173	25,136
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	307,038	256,746
Gross Unrealized Gains	7,025	6,019
Gross Unrealized Losses	(53)	0
Estimated Fair Value	314,010	262,765
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	95,389	100,336
Gross Unrealized Gains	2,773	2,695
Gross Unrealized Losses	(8,276)	(6,076)
Estimated Fair Value	89,886	96,955
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	782,018	835,439
Gross Unrealized Gains	5,322	3,042
Gross Unrealized Losses	(5,265)	(16,972)
Estimated Fair Value	782,075	821,509
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	1,169,552	1,154,935
Gross Unrealized Gains	19,454	10,104
Gross Unrealized Losses	(338)	(6,204)
Estimated Fair Value	1,188,668	1,158,835
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	106,871	106,772
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(9,545)	(8,713)
Estimated Fair Value	97,326	98,059
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	2,486,017	2,479,382
Gross Unrealized Gains	34,598	21,895
Gross Unrealized Losses	(23,477)	(38,018)
Estimated Fair Value	$ 2,497,138	$ 2,463,259