Investments with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale, Fair Value
Less than 12 Months	$ 1,163,595	$ 358,126
12 Months or More	182,928	202,027
Total	1,346,523	560,153
Available-for-sale, Unrealized Loss
Less than 12 Months	(11,262)	(2,746)
12 Months or Longer	(7,056)	(5,061)
Total	(18,318)	(7,807)
Held-to-maturity, Fair Value
Less than 12 Months	42,322	18,643
12 Months or More	81,709	76,761
Total	124,031	95,404
Held-to-maturity, Unrealized Loss
Less than 12 Months	(594)	(624)
12 Months or More	(3,828)	(2,352)
Total	(4,422)	(2,976)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	42,493	0
12 Months or More	2	3
Total	42,495	3
Available-for-sale, Unrealized Loss
Less than 12 Months	(432)	0
12 Months or Longer	0	0
Total	(432)	0
U.S. government sponsored enterprises
Available-for-sale, Fair Value
Less than 12 Months	60,010	13,793
12 Months or More	0	0
Total	60,010	13,793
Available-for-sale, Unrealized Loss
Less than 12 Months	(163)	(2)
12 Months or Longer	0	0
Total	(163)	(2)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	102,422	91,082
12 Months or More	115,943	115,927
Total	218,365	207,009
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,629)	(1,273)
12 Months or Longer	(4,249)	(3,296)
Total	(5,878)	(4,569)
Held-to-maturity, Fair Value
Less than 12 Months	42,322	18,643
12 Months or More	81,709	76,761
Total	124,031	95,404
Held-to-maturity, Unrealized Loss
Less than 12 Months	(594)	(624)
12 Months or More	(3,828)	(2,352)
Total	(4,422)	(2,976)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	228,258	60,289
12 Months or More	13,962	7,874
Total	242,220	68,163
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,812)	(545)
12 Months or Longer	(1,265)	(205)
Total	(3,077)	(750)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	730,412	192,962
12 Months or More	53,021	78,223
Total	783,433	271,185
Available-for-sale, Unrealized Loss
Less than 12 Months	(7,226)	(926)
12 Months or Longer	(1,542)	(1,560)
Total	$ (8,768)	$ (2,486)