Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale, Fair Value
Less than 12 Months	$ 116,496	$ 1,163,595
12 Months or More	204,345	182,928
Total	320,841	1,346,523
Available-for-sale, Unrealized Loss
Less than 12 Months	(609)	(11,262)
12 Months or More	(5,690)	(7,056)
Total	(6,299)	(18,318)
Held-to-maturity, Fair Value
Less than 12 Months	2,967	42,322
12 Months or More	90,809	81,709
Total	93,776	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(73)	(594)
12 Months or More	(3,116)	(3,828)
Total	(3,189)	(4,422)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	1,686	42,493
12 Months or More	34,603	2
Total	36,289	42,495
Available-for-sale, Unrealized Loss
Less than 12 Months	(2)	(432)
12 Months or More	(218)	0
Total	(220)	(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	15,644	102,422
12 Months or More	123,203	115,943
Total	138,847	218,365
Available-for-sale, Unrealized Loss
Less than 12 Months	(239)	(1,629)
12 Months or More	(5,096)	(4,249)
Total	(5,335)	(5,878)
Held-to-maturity, Fair Value
Less than 12 Months	2,967	42,322
12 Months or More	90,809	81,709
Total	93,776	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(73)	(594)
12 Months or More	(3,116)	(3,828)
Total	(3,189)	(4,422)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	36,068	228,258
12 Months or More	18,521	13,962
Total	54,589	242,220
Available-for-sale, Unrealized Loss
Less than 12 Months	(97)	(1,812)
12 Months or More	(95)	(1,265)
Total	(192)	(3,077)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	63,098	730,412
12 Months or More	28,018	53,021
Total	91,116	783,433
Available-for-sale, Unrealized Loss
Less than 12 Months	(271)	(7,226)
12 Months or More	(281)	(1,542)
Total	$ (552)	$ (8,768)