Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale, Fair Value
Less than 12 Months	$ 128,027	$ 1,163,595
12 Months or More	168,550	182,928
Total	296,577	1,346,523
Available-for-sale, Unrealized Loss
Less than 12 Months	(705)	(11,262)
12 Months or More	(5,677)	(7,056)
Total	(6,382)	(18,318)
Held-to-maturity, Fair Value
Less than 12 Months	13,863	42,322
12 Months or More	89,024	81,709
Total	102,887	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(159)	(594)
12 Months or More	(4,195)	(3,828)
Total	(4,354)	(4,422)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	2,190	42,493
12 Months or More	30,009	2
Total	32,199	42,495
Available-for-sale, Unrealized Loss
Less than 12 Months	(7)	(432)
12 Months or More	(151)	0
Total	(158)	(432)
U.S. government sponsored enterprises
Available-for-sale, Fair Value
Less than 12 Months		60,010
12 Months or More		0
Total		60,010
Available-for-sale, Unrealized Loss
Less than 12 Months		(163)
12 Months or More		0
Total		(163)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	45,697	102,422
12 Months or More	109,147	115,943
Total	154,844	218,365
Available-for-sale, Unrealized Loss
Less than 12 Months	(443)	(1,629)
12 Months or More	(5,321)	(4,249)
Total	(5,764)	(5,878)
Held-to-maturity, Fair Value
Less than 12 Months	13,863	42,322
12 Months or More	89,024	81,709
Total	102,887	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(159)	(594)
12 Months or More	(4,195)	(3,828)
Total	(4,354)	(4,422)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	5,049	228,258
12 Months or More	6,153	13,962
Total	11,202	242,220
Available-for-sale, Unrealized Loss
Less than 12 Months	(3)	(1,812)
12 Months or More	(29)	(1,265)
Total	(32)	(3,077)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	75,091	730,412
12 Months or More	23,241	53,021
Total	98,332	783,433
Available-for-sale, Unrealized Loss
Less than 12 Months	(252)	(7,226)
12 Months or More	(176)	(1,542)
Total	$ (428)	$ (8,768)