Investment Securities (Details 4) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Held To Maturity Securities Continuous Unrealized Loss Position [Abstract]
Held to maturity, Estimated fair value, Less than 12 months	$ 4,333	$ 76,318
Held to maturity, Unrealized Losses, Less than 12 months	(1,127)	(638)
Held to maturity, Estimated fair value, 12 months or more	175,774	198,607
Held to maturity, Unrealized Losses, 12 months or more	(98,944)	(114,453)
Investment securities in continuous unrealized loss position
Total investment securities, fair value less than 12 months	435,309	256,940
Total of investment securities, unrealized losses, less than 12 months	(18,227)	(13,497)
Total of investment securities, fair value, more than 12 months	1,079,651	1,379,002
Total of investment securities, unrealized losses, more than 12 months	(364,057)	(413,331)
Obligations of states and political subdivisions [Member]
Held To Maturity Securities Continuous Unrealized Loss Position [Abstract]
Held to maturity, Estimated fair value, Less than 12 months	1,542	76,318
Held to maturity, Unrealized Losses, Less than 12 months	(11)	(638)
Held to maturity, Estimated fair value, 12 months or more	184	467
Held to maturity, Unrealized Losses, 12 months or more	(8)	(56)
Privately Issued Mortgage Backed Securities [Member]
Held To Maturity Securities Continuous Unrealized Loss Position [Abstract]
Held to maturity, Estimated fair value, Less than 12 months	2,791	0
Held to maturity, Unrealized Losses, Less than 12 months	(1,116)	0
Held to maturity, Estimated fair value, 12 months or more	175,590	198,140
Held to maturity, Unrealized Losses, 12 months or more	$ (98,936)	$ (114,397)