Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Amortized Cost	$ 2,473,788	$ 2,608,210
Gross Unrealized Gains	45,528	46,681
Gross Unrealized Losses	(48,707)	(86,457)
Estimated Fair Value	2,470,609	2,568,434
Equity Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	34,380	33,922
Gross Unrealized Gains	10,927	12,355
Gross Unrealized Losses	(29)	(76)
Estimated Fair Value	45,278	46,201
U.S. Government Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	200	525
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	200	525
U.S. Government-Sponsored Agency Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	235	720
Gross Unrealized Gains	5	7
Gross Unrealized Losses	0	(1)
Estimated Fair Value	240	726
State and Municipal Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	250,195	281,810
Gross Unrealized Gains	7,917	6,483
Gross Unrealized Losses	(496)	(3,444)
Estimated Fair Value	257,616	284,849
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	99,670	100,468
Gross Unrealized Gains	5,777	5,685
Gross Unrealized Losses	(4,020)	(7,404)
Estimated Fair Value	101,427	98,749
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	975,971	1,069,138
Gross Unrealized Gains	6,700	8,036
Gross Unrealized Losses	(27,631)	(44,776)
Estimated Fair Value	955,040	1,032,398
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	954,412	949,328
Gross Unrealized Gains	14,201	13,881
Gross Unrealized Losses	(6,278)	(17,497)
Estimated Fair Value	962,335	945,712
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	158,725	172,299
Gross Unrealized Gains	1	234
Gross Unrealized Losses	(10,253)	(13,259)
Estimated Fair Value	$ 148,473	$ 159,274