Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale, Fair Value
Less than 12 Months	$ 663,604	$ 1,188,397
12 Months or More	90,525	144,120
Total	754,129	1,332,517
Available-for-sale, Unrealized Loss
Less than 12 Months	(4,783)	(16,446)
12 Months or More	(2,537)	(5,839)
Total	(7,320)	(22,285)
Held-to-maturity, Fair Value
Less than 12 Months	32,640	117,912
12 Months or More	82,953	86,601
Total	115,593	204,513
Held-to-maturity, Unrealized Loss
Less than 12 Months	(642)	(1,712)
12 Months or More	(6,255)	(6,273)
Total	(6,897)	(7,985)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	3,799	6,718
12 Months or More	20,347	26,239
Total	24,146	32,957
Available-for-sale, Unrealized Loss
Less than 12 Months	(23)	(24)
12 Months or More	(170)	(138)
Total	(193)	(162)
U.S. government sponsored enterprises
Available-for-sale, Fair Value
Less than 12 Months	3,460	6,049
12 Months or More	0	0
Total	3,460	6,049
Available-for-sale, Unrealized Loss
Less than 12 Months	(9)	(42)
12 Months or More	0	0
Total	(9)	(42)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	108,608	222,700
12 Months or More	46,043	81,783
Total	154,651	304,483
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,436)	(4,949)
12 Months or More	(2,157)	(5,014)
Total	(3,593)	(9,963)
Held-to-maturity, Fair Value
Less than 12 Months	32,640	117,912
12 Months or More	82,953	86,601
Total	115,593	204,513
Held-to-maturity, Unrealized Loss
Less than 12 Months	(642)	(1,712)
12 Months or More	(6,255)	(6,273)
Total	(6,897)	(7,985)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	114,851	174,821
12 Months or More	6,045	6,141
Total	120,896	180,962
Available-for-sale, Unrealized Loss
Less than 12 Months	(254)	(774)
12 Months or More	(8)	(19)
Total	(262)	(793)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	363,572	688,811
12 Months or More	18,090	29,957
Total	381,662	718,768
Available-for-sale, Unrealized Loss
Less than 12 Months	(2,576)	(9,079)
12 Months or More	(202)	(668)
Total	(2,778)	(9,747)
Commercial mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	69,314	89,298
12 Months or More	0	0
Total	69,314	89,298
Available-for-sale, Unrealized Loss
Less than 12 Months	(485)	(1,578)
12 Months or More	0	0
Total	$ (485)	$ (1,578)