Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale, Fair Value
Less than 12 Months	$ 540,180	$ 358,126
12 Months or More	204,995	202,027
Total	745,175	560,153
Available-for-sale, Unrealized Loss
Less than 12 Months	(4,544)	(2,746)
12 Months or More	(6,220)	(5,061)
Total	(10,764)	(7,807)
Held-to-maturity, Fair Value
Less than 12 Months	108,671	18,643
12 Months or More	69,625	76,761
Total	178,296	95,404
Held-to-maturity, Unrealized Loss
Less than 12 Months	(2,389)	(624)
12 Months or More	(3,508)	(2,352)
Total	(5,897)	(2,976)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	46,129	0
12 Months or More	2	3
Total	46,131	3
Available-for-sale, Unrealized Loss
Less than 12 Months	(412)	0
12 Months or More	0	0
Total	(412)	0
U.S. government sponsored enterprises
Available-for-sale, Fair Value
Less than 12 Months	4,595	13,793
12 Months or More	0	0
Total	4,595	13,793
Available-for-sale, Unrealized Loss
Less than 12 Months	(3)	(2)
12 Months or More	0	0
Total	(3)	(2)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	135,878	91,082
12 Months or More	137,894	115,927
Total	273,772	207,009
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,986)	(1,273)
12 Months or More	(4,820)	(3,296)
Total	(6,806)	(4,569)
Held-to-maturity, Fair Value
Less than 12 Months	108,671	18,643
12 Months or More	69,625	76,761
Total	178,296	95,404
Held-to-maturity, Unrealized Loss
Less than 12 Months	(2,389)	(624)
12 Months or More	(3,508)	(2,352)
Total	(5,897)	(2,976)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	131,483	60,289
12 Months or More	10,736	7,874
Total	142,219	68,163
Available-for-sale, Unrealized Loss
Less than 12 Months	(826)	(545)
12 Months or More	(428)	(205)
Total	(1,254)	(750)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	222,095	192,962
12 Months or More	56,363	78,223
Total	278,458	271,185
Available-for-sale, Unrealized Loss
Less than 12 Months	(1,317)	(926)
12 Months or More	(972)	(1,560)
Total	$ (2,289)	$ (2,486)