Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Amortized Cost	$ 2,237,100	$ 2,312,087
Gross Unrealized Gains	49,610	49,212
Gross Unrealized Losses	(26,908)	(37,928)
Estimated Fair Value	2,259,802us-gaap_AvailableForSaleSecurities	2,323,371us-gaap_AvailableForSaleSecurities
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	29,224	33,469
Gross Unrealized Gains	11,397	14,167
Gross Unrealized Losses	(13)	(13)
Estimated Fair Value	40,608	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	198	209
Gross Unrealized Gains	5	5
Gross Unrealized Losses	0	0
Estimated Fair Value	203	214
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	216,877	238,250
Gross Unrealized Gains	7,496	7,231
Gross Unrealized Losses	(31)	(266)
Estimated Fair Value	224,342	245,215
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	99,120	99,016
Gross Unrealized Gains	3,929	5,126
Gross Unrealized Losses	(5,367)	(6,108)
Estimated Fair Value	97,682	98,034
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	874,853	917,395
Gross Unrealized Gains	5,924	5,705
Gross Unrealized Losses	(12,901)	(20,787)
Estimated Fair Value	867,876	902,313
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	910,418	914,797
Gross Unrealized Gains	20,859	16,978
Gross Unrealized Losses	(1,118)	(2,944)
Estimated Fair Value	930,159	928,831
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	106,410	108,751
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7,478)	(7,810)
Estimated Fair Value	$ 98,932	$ 100,941

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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