Investment with an Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value
Fair Value, Less than 12 Months	$ 1,041,606	$ 1,099,987
Fair Value, 12 Months or More	22,373	67,438
Fair Value, Total	1,063,979	1,167,425
Unrealized Loss
Unrealized Loss, Less than 12 Months	(31,975)	(5,810)
Unrealized Loss, 12 Months or More	(1,746)	(640)
Unrealized Loss, Total	(33,721)	(6,450)
State and local governments
Fair Value
Fair Value, Less than 12 Months	405,834	102,896
Fair Value, 12 Months or More	22,373	4,533
Fair Value, Total	428,207	107,429
Unrealized Loss
Unrealized Loss, Less than 12 Months	(21,121)	(1,531)
Unrealized Loss, 12 Months or More	(1,746)	(74)
Unrealized Loss, Total	(22,867)	(1,605)
Corporate bonds
Fair Value
Fair Value, Less than 12 Months	189,130	41,856
Fair Value, 12 Months or More	0	0
Fair Value, Total	189,130	41,856
Unrealized Loss
Unrealized Loss, Less than 12 Months	(2,937)	(238)
Unrealized Loss, 12 Months or More	0	0
Unrealized Loss, Total	(2,937)	(238)
Residential mortgage-backed securities
Fair Value
Fair Value, Less than 12 Months	446,642	955,235
Fair Value, 12 Months or More	0	62,905
Fair Value, Total	446,642	1,018,140
Unrealized Loss
Unrealized Loss, Less than 12 Months	(7,917)	(4,041)
Unrealized Loss, 12 Months or More	0	(566)
Unrealized Loss, Total	$ (7,917)	$ (4,607)