Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Amortized Cost	$ 2,438,677	$ 2,312,087
Gross Unrealized Gains	36,668	49,212
Gross Unrealized Losses	(34,853)	(37,928)
Estimated Fair Value	2,440,492	2,323,371
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	23,981	33,469
Gross Unrealized Gains	9,072	14,167
Gross Unrealized Losses	(12)	(13)
Estimated Fair Value	33,041	47,623
U.S. Government Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost		200
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		200
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	48,333	209
Gross Unrealized Gains	57	5
Gross Unrealized Losses	(130)	0
Estimated Fair Value	48,260	214
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	231,592	238,250
Gross Unrealized Gains	5,312	7,231
Gross Unrealized Losses	(387)	(266)
Estimated Fair Value	236,517	245,215
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	98,756	99,016
Gross Unrealized Gains	3,183	5,126
Gross Unrealized Losses	(4,767)	(6,108)
Estimated Fair Value	97,172	98,034
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	931,093	917,395
Gross Unrealized Gains	4,932	5,705
Gross Unrealized Losses	(17,793)	(20,787)
Estimated Fair Value	918,232	902,313
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	998,418	914,797
Gross Unrealized Gains	14,112	16,978
Gross Unrealized Losses	(3,866)	(2,944)
Estimated Fair Value	1,008,664	928,831
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	106,504	108,751
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7,898)	(7,810)
Estimated Fair Value	$ 98,606	$ 100,941