Investments with an Unrealized Loss Position (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value
Less than 12 Months	$ 358,126	$ 995,941
12 Months or More	202,027	77,856
Total	560,153	1,073,797
Unrealized Loss
Less than 12 Months	(2,746)	(29,736)
12 Months or Longer	(5,061)	(6,177)
Total	(7,807)	(35,913)
Fair Value
Less than 12 Months	18,643
12 Months or More	76,761
Total	95,404
Unrealized Loss
Less than 12 Months	(624)
12 Months or More	(2,352)
Total	(2,976)
U.S. government and federal agency
Fair Value
Less than 12 Months	0
12 Months or More	3
Total	3
Unrealized Loss
Less than 12 Months	0
12 Months or Longer	0
Total	0
U.S. government sponsored enterprises
Fair Value
Less than 12 Months	13,793	3
12 Months or More	0	0
Total	13,793	3
Unrealized Loss
Less than 12 Months	(2)	0
12 Months or Longer	0	0
Total	(2)	0
State and local governments
Fair Value
Less than 12 Months	91,082	408,812
12 Months or More	115,927	74,161
Total	207,009	482,973
Unrealized Loss
Less than 12 Months	(1,273)	(17,838)
12 Months or Longer	(3,296)	(6,052)
Total	(4,569)	(23,890)
Fair Value
Less than 12 Months	18,643
12 Months or More	76,761
Total	95,404
Unrealized Loss
Less than 12 Months	(624)
12 Months or More	(2,352)
Total	(2,976)
Corporate bonds
Fair Value
Less than 12 Months	60,289	129,515
12 Months or More	7,874	1,702
Total	68,163	131,217
Unrealized Loss
Less than 12 Months	(545)	(1,672)
12 Months or Longer	(205)	(86)
Total	(750)	(1,758)
Residential mortgage-backed securities
Fair Value
Less than 12 Months	192,962	457,611
12 Months or More	78,223	1,993
Total	271,185	459,604
Unrealized Loss
Less than 12 Months	(926)	(10,226)
12 Months or Longer	(1,560)	(39)
Total	$ (2,486)	$ (10,265)