Investments with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale, Fair Value
Less than 12 Months	$ 1,188,397	$ 1,163,595
12 Months or More	144,120	182,928
Total	1,332,517	1,346,523
Available-for-sale, Unrealized Loss
Less than 12 Months	(16,446)	(11,262)
12 Months or Longer	(5,839)	(7,056)
Total	(22,285)	(18,318)
Held-to-maturity, Fair Value
Less than 12 Months	117,912	42,322
12 Months or More	86,601	81,709
Total	204,513	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(1,712)	(594)
12 Months or More	(6,273)	(3,828)
Total	(7,985)	(4,422)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	6,718	42,493
12 Months or More	26,239	2
Total	32,957	42,495
Available-for-sale, Unrealized Loss
Less than 12 Months	(24)	(432)
12 Months or Longer	(138)	0
Total	(162)	(432)
U.S. government sponsored enterprises
Available-for-sale, Fair Value
Less than 12 Months	6,049	60,010
12 Months or More	0	0
Total	6,049	60,010
Available-for-sale, Unrealized Loss
Less than 12 Months	(42)	(163)
12 Months or Longer	0	0
Total	(42)	(163)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	222,700	102,422
12 Months or More	81,783	115,943
Total	304,483	218,365
Available-for-sale, Unrealized Loss
Less than 12 Months	(4,949)	(1,629)
12 Months or Longer	(5,014)	(4,249)
Total	(9,963)	(5,878)
Held-to-maturity, Fair Value
Less than 12 Months	117,912	42,322
12 Months or More	86,601	81,709
Total	204,513	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(1,712)	(594)
12 Months or More	(6,273)	(3,828)
Total	(7,985)	(4,422)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	174,821	228,258
12 Months or More	6,141	13,962
Total	180,962	242,220
Available-for-sale, Unrealized Loss
Less than 12 Months	(774)	(1,812)
12 Months or Longer	(19)	(1,265)
Total	(793)	(3,077)
Mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	778,109	730,412
12 Months or More	29,957	53,021
Total	808,066	783,433
Available-for-sale, Unrealized Loss
Less than 12 Months	(10,657)	(7,226)
12 Months or Longer	(668)	(1,542)
Total	$ (11,325)	$ (8,768)