Investment Securities (Schedule of Gross Unrealized Losses and Fair Values of Securities) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Less than 12 months, Fair Value	$ 1,098,087	$ 1,821,091
Less than 12 months, Gross Unrealized Losses	(15,264)	(44,501)
12 months or more, Fair Value	131,556	170,536
12 months or more, Gross Unrealized Losses	(7,439)	(17,506)
Total Fair Value	1,229,643	1,991,627
Total Gross Unrealized Losses	(22,703)	(62,007)
Debt Securities [Member] | U.S. Treasury and Government [Member] | U.S. Treasury and Agency Obligations [Member]
Less than 12 months, Fair Value	74,009	141,766
Less than 12 months, Gross Unrealized Losses	(440)	(615)
12 months or more, Fair Value	0	0
12 months or more, Gross Unrealized Losses	0	0
Total Fair Value	74,009	141,766
Total Gross Unrealized Losses	(440)	(615)
Debt Securities [Member] | U.S. Treasury and Government [Member] | Mortgage-Backed Securities [Member]
Less than 12 months, Fair Value	40,023	110,358
Less than 12 months, Gross Unrealized Losses	(422)	(1,054)
12 months or more, Fair Value	21,650	5,557
12 months or more, Gross Unrealized Losses	(338)	(119)
Total Fair Value	61,673	115,915
Total Gross Unrealized Losses	(760)	(1,173)
Debt Securities [Member] | Tax-Exempt Municipal Securities [Member]
Less than 12 months, Fair Value	480,854	1,168,221
Less than 12 months, Gross Unrealized Losses	(6,930)	(33,218)
12 months or more, Fair Value	63,336	97,809
12 months or more, Gross Unrealized Losses	(3,808)	(10,401)
Total Fair Value	544,190	1,266,030
Total Gross Unrealized Losses	(10,738)	(43,619)
Debt Securities [Member] | Residential Mortgage-Backed Securities [Member]
Less than 12 months, Fair Value	3,605	0
Less than 12 months, Gross Unrealized Losses	(59)	0
12 months or more, Fair Value	26,807	32,671
12 months or more, Gross Unrealized Losses	(1,475)	(2,675)
Total Fair Value	30,412	32,671
Total Gross Unrealized Losses	(1,534)	(2,675)
Debt Securities [Member] | Commercial Mortgage-Backed Securities [Member]
Less than 12 months, Fair Value	96,425	0
Less than 12 months, Gross Unrealized Losses	(905)	0
12 months or more, Fair Value	156	2,752
12 months or more, Gross Unrealized Losses	(12)	(171)
Total Fair Value	96,581	2,752
Total Gross Unrealized Losses	(917)	(171)
Debt Securities [Member] | Asset-Backed Securities [Member]
Less than 12 months, Fair Value	4,860	17,069
Less than 12 months, Gross Unrealized Losses	(9)	(42)
12 months or more, Fair Value	5,059	283
12 months or more, Gross Unrealized Losses	(20)	(2)
Total Fair Value	9,919	17,352
Total Gross Unrealized Losses	(29)	(44)
Debt Securities [Member] | Corporate Debt Securities [Member]
Less than 12 months, Fair Value	398,311	383,677
Less than 12 months, Gross Unrealized Losses	(6,499)	(9,572)
12 months or more, Fair Value	14,548	31,464
12 months or more, Gross Unrealized Losses	(1,786)	(4,138)
Total Fair Value	412,859	415,141
Total Gross Unrealized Losses	$ (8,285)	$ (13,710)