Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale, Fair Value
Less than 12 Months	$ 539,859	$ 1,188,397
12 Months or More	147,626	144,120
Total	687,485	1,332,517
Available-for-sale, Unrealized Loss
Less than 12 Months	(3,920)	(16,446)
12 Months or More	(3,673)	(5,839)
Total	(7,593)	(22,285)
Held-to-maturity, Fair Value
Less than 12 Months	10,683	117,912
12 Months or More	104,858	86,601
Total	115,541	204,513
Held-to-maturity, Unrealized Loss
Less than 12 Months	(139)	(1,712)
12 Months or More	(7,294)	(6,273)
Total	(7,433)	(7,985)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	1,952	6,718
12 Months or More	13,548	26,239
Total	15,500	32,957
Available-for-sale, Unrealized Loss
Less than 12 Months	(18)	(24)
12 Months or More	(142)	(138)
Total	(160)	(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	(11)	(42)
12 Months or More	0	0
Total	(11)	(42)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	76,806	222,700
12 Months or More	83,941	81,783
Total	160,747	304,483
Available-for-sale, Unrealized Loss
Less than 12 Months	(932)	(4,949)
12 Months or More	(3,007)	(5,014)
Total	(3,939)	(9,963)
Held-to-maturity, Fair Value
Less than 12 Months	10,683	117,912
12 Months or More	104,858	86,601
Total	115,541	204,513
Held-to-maturity, Unrealized Loss
Less than 12 Months	(139)	(1,712)
12 Months or More	(7,294)	(6,273)
Total	(7,433)	(7,985)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	48,048	174,821
12 Months or More	3,270	6,141
Total	51,318	180,962
Available-for-sale, Unrealized Loss
Less than 12 Months	(150)	(774)
12 Months or More	(6)	(19)
Total	(156)	(793)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	350,965	688,811
12 Months or More	38,751	29,957
Total	389,716	718,768
Available-for-sale, Unrealized Loss
Less than 12 Months	(2,102)	(9,079)
12 Months or More	(480)	(668)
Total	(2,582)	(9,747)
Commercial mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	58,628	89,298
12 Months or More	8,116	0
Total	66,744	89,298
Available-for-sale, Unrealized Loss
Less than 12 Months	(707)	(1,578)
12 Months or More	(38)	0
Total	$ (745)	$ (1,578)