Investment Securities, Available-for-Sale (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value
Fair Value, Less than 12 Months	$ 1,035,710	$ 1,099,987
Fair Value, 12 Months or More	26,425	67,438
Fair Value, Total	1,062,135	1,167,425
Unrealized Loss
Unrealized Loss, Less than 12 Months	(25,592)	(5,810)
Unrealized Loss, 12 Months or More	(443)	(640)
Unrealized Loss, Total	(26,035)	(6,450)
State and local governments [Member]
Fair Value
Fair Value, Less than 12 Months	312,576	102,896
Fair Value, 12 Months or More	10,434	4,533
Fair Value, Total	323,010	107,429
Unrealized Loss
Unrealized Loss, Less than 12 Months	(15,987)	(1,531)
Unrealized Loss, 12 Months or More	(400)	(74)
Unrealized Loss, Total	(16,387)	(1,605)
Corporate bonds [Member]
Fair Value
Fair Value, Less than 12 Months	211,592	41,856
Fair Value, 12 Months or More	0
Fair Value, Total	211,592	41,856
Unrealized Loss
Unrealized Loss, Less than 12 Months	(3,519)	(238)
Unrealized Loss, 12 Months or More	0
Unrealized Loss, Total	(3,519)	(238)
Residential mortgage-backed securities [Member]
Fair Value
Fair Value, Less than 12 Months	511,542	955,235
Fair Value, 12 Months or More	15,991	62,905
Fair Value, Total	527,533	1,018,140
Unrealized Loss
Unrealized Loss, Less than 12 Months	(6,086)	(4,041)
Unrealized Loss, 12 Months or More	(43)	(566)
Unrealized Loss, Total	$ (6,129)	$ (4,607)