Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available-for-sale, Fair Value
Less than 12 Months	$ 520,654	$ 1,163,595
12 Months or More	284,198	182,928
Total	804,852	1,346,523
Available-for-sale, Unrealized Loss
Less than 12 Months	(3,331)	(11,262)
12 Months or More	(6,968)	(7,056)
Total	(10,299)	(18,318)
Held-to-maturity, Fair Value
Less than 12 Months	16,230	42,322
12 Months or More	108,674	81,709
Total	124,904	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(209)	(594)
12 Months or More	(4,345)	(3,828)
Total	(4,554)	(4,422)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	2,291	42,493
12 Months or More	39,478	2
Total	41,769	42,495
Available-for-sale, Unrealized Loss
Less than 12 Months	(5)	(432)
12 Months or More	(283)	0
Total	(288)	(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	50,820	102,422
12 Months or More	145,853	115,943
Total	196,673	218,365
Available-for-sale, Unrealized Loss
Less than 12 Months	(444)	(1,629)
12 Months or More	(5,382)	(4,249)
Total	(5,826)	(5,878)
Held-to-maturity, Fair Value
Less than 12 Months	16,230	42,322
12 Months or More	108,674	81,709
Total	124,904	124,031
Held-to-maturity, Unrealized Loss
Less than 12 Months	(209)	(594)
12 Months or More	(4,345)	(3,828)
Total	(4,554)	(4,422)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	208,048	228,258
12 Months or More	33,786	13,962
Total	241,834	242,220
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,227)	(1,812)
12 Months or More	(268)	(1,265)
Total	(1,495)	(3,077)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	259,495	730,412
12 Months or More	65,081	53,021
Total	324,576	783,433
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,655)	(7,226)
12 Months or More	(1,035)	(1,542)
Total	$ (2,690)	$ (8,768)