Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Available-for-sale securities before taxes:
Change in net unrealized gain/loss during the period	$ 20,955	$ 9,320
Reclassification adjustment for net realized gain on sale of available-for-sale securities included in net income	(232)	(290)
Net unrealized gains	20,723	9,030
Available-for-sale securities taxes:
Change in net unrealized gain/loss during the period	(8,380)	(3,728)
Reclassification adjustment for net realized gain on sale of available-for-sale securities included in net income	93	116
Net unrealized gains	(8,287)	(3,612)
Available-for-sale securities net of taxes:
Change in net unrealized gain/loss during the period	12,575	5,592
Reclassification adjustment for net realized gain on sale of available-for-sale securities included in net income	(139)	(174)
Net unrealized gains	12,436	5,418
Employee benefit plans before taxes:
Amortization of net retirement plan actuarial gain	343	731
Amortization of net retirement plan prior service (credit) cost	30	(179)
Employee benefit plans	373	552
Other comprehensive income	21,096	9,582
Employee benefit plans taxes:
Amortization of net retirement plan actuarial gain	(137)	(293)
Amortization of net retirement plan prior service (credit) cost	(12)	72
Employee benefit plans	(149)	(221)
Other comprehensive income	(8,436)	(3,833)
Employee benefit plans net of taxes:
Amortization of net retirement plan actuarial gain	206	438
Amortization of net retirement plan prior service (credit) cost	18	(107)
Employee benefit plans	224	331
Other comprehensive income	$ 12,660	$ 5,749