Other Comprehensive Income - Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in net unrealized Gain during period:
Change in net unrealized gain during period, before tax amount	$ 21,147	$ (52,949)	$ (4,648)
Change in net unrealized gain during period, tax expense (benefit)	8,321	(20,835)	(1,829)
Change in net unrealized gain during period, net of tax amount	12,826	(32,114)	(2,819)
Investment securities available-for-sale:
Reclassification adjustment for net gains included in net income, before tax amount	(61)	(1)	(348)
Reclassification adjustment for net, gains included in net income, tax expense (benefit)	(24)	0	(137)
Reclassification adjustment for net gains included in net income, net of tax amount	(37)	(1)	(211)
Unamortized Premium on Available-For-Sale Securities Transferred to Held-To-Maturity
Unamortized premium on available-to-sale securities transferred into held-for-maturity, before tax amount	(548)		56
Unamortized premium on available-to-sale securities transferred into held-for-maturity, tax expense (benefit)	(216)		22
Unamortized premium on available-to-sale securities transferred into held-for-maturity, net of tax amount	(332)		34
Change in net actuarial loss:
Change in net actuarial loss, before tax amount	1,731	137	(77)
Change in net actuarial loss, tax expense (benefit)	681	54	(30)
Change in net actuarial loss, net of tax amount	1,050	83	(47)
Other comprehensive income (Loss)
Other comprehensive income (loss), before tax	22,269	(52,813)	(5,017)
Total other comprehensive income, tax expense (benefit)	8,762	(20,781)	(1,974)
Other comprehensive income, net of tax	$ 13,507	$ (32,032)	$ (3,043)