Investments with an Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value
Fair Value, Less than 12 Months	$ 995,941	$ 1,099,987
Fair Value, 12 Months or More	77,856	67,438
Fair Value, Total	1,073,797	1,167,425
Unrealized Loss
Unrealized Loss, Less than 12 Months	(29,736)	(5,810)
Unrealized Loss, 12 Months or Longer	(6,177)	(640)
Unrealized Loss, Total	(35,913)	(6,450)
U.S. government sponsored enterprises
Fair Value
Fair Value, Less than 12 Months	3
Fair Value, 12 Months or More	0
Fair Value, Total	3
Unrealized Loss
Unrealized Loss, Less than 12 Months	0
Unrealized Loss, 12 Months or Longer	0
Unrealized Loss, Total	0
State and local governments
Fair Value
Fair Value, Less than 12 Months	408,812	102,896
Fair Value, 12 Months or More	74,161	4,533
Fair Value, Total	482,973	107,429
Unrealized Loss
Unrealized Loss, Less than 12 Months	(17,838)	(1,531)
Unrealized Loss, 12 Months or Longer	(6,052)	(74)
Unrealized Loss, Total	(23,890)	(1,605)
Corporate bonds
Fair Value
Fair Value, Less than 12 Months	129,515	41,856
Fair Value, 12 Months or More	1,702	0
Fair Value, Total	131,217	41,856
Unrealized Loss
Unrealized Loss, Less than 12 Months	(1,672)	(238)
Unrealized Loss, 12 Months or Longer	(86)	0
Unrealized Loss, Total	(1,758)	(238)
Residential mortgage-backed securities
Fair Value
Fair Value, Less than 12 Months	457,611	955,235
Fair Value, 12 Months or More	1,993	62,905
Fair Value, Total	459,604	1,018,140
Unrealized Loss
Unrealized Loss, Less than 12 Months	(10,226)	(4,041)
Unrealized Loss, 12 Months or Longer	(39)	(566)
Unrealized Loss, Total	$ (10,265)	$ (4,607)