Investment with an Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available-for-sale, Fair Value
Less than 12 Months	$ 740,350	$ 358,126
12 Months or More	158,598	202,027
Total	898,948	560,153
Available-for-sale, Unrealized Loss
Less than 12 Months	(10,822)	(2,746)
12 Months or More	(6,176)	(5,061)
Total	(16,998)	(7,807)
Held-to-maturity, Fair Value
Less than 12 Months	126,065	18,643
12 Months or More	65,860	76,761
Total	191,925	95,404
Held-to-maturity, Unrealized Loss
Less than 12 Months	(4,113)	(624)
12 Months or More	(3,728)	(2,352)
Total	(7,841)	(2,976)
U.S. government and federal agency
Available-for-sale, Fair Value
Less than 12 Months	48,276	0
12 Months or More	3	3
Total	48,279	3
Available-for-sale, Unrealized Loss
Less than 12 Months	(405)	0
12 Months or More	0	0
Total	(405)	0
U.S. government sponsored enterprises
Available-for-sale, Fair Value
Less than 12 Months	64,511	13,793
12 Months or More	0	0
Total	64,511	13,793
Available-for-sale, Unrealized Loss
Less than 12 Months	(356)	(2)
12 Months or More	0	0
Total	(356)	(2)
State and local governments
Available-for-sale, Fair Value
Less than 12 Months	228,286	91,082
12 Months or More	106,178	115,927
Total	334,464	207,009
Available-for-sale, Unrealized Loss
Less than 12 Months	(5,018)	(1,273)
12 Months or More	(4,328)	(3,296)
Total	(9,346)	(4,569)
Held-to-maturity, Fair Value
Less than 12 Months	126,065	18,643
12 Months or More	65,860	76,761
Total	191,925	95,404
Held-to-maturity, Unrealized Loss
Less than 12 Months	(4,113)	(624)
12 Months or More	(3,728)	(2,352)
Total	(7,841)	(2,976)
Corporate bonds
Available-for-sale, Fair Value
Less than 12 Months	128,016	60,289
12 Months or More	3,635	7,874
Total	131,651	68,163
Available-for-sale, Unrealized Loss
Less than 12 Months	(736)	(545)
12 Months or More	(37)	(205)
Total	(773)	(750)
Residential mortgage-backed securities
Available-for-sale, Fair Value
Less than 12 Months	271,261	192,962
12 Months or More	48,782	78,223
Total	320,043	271,185
Available-for-sale, Unrealized Loss
Less than 12 Months	(4,307)	(926)
12 Months or More	(1,811)	(1,560)
Total	$ (6,118)	$ (2,486)