Other Comprehensive Income (Loss) - Tax Effect Allocated to Each Component of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Before-Tax Amount
Other Comprehensive Income, Net unrealized losses	$ (6,097)	$ (10,176)	$ (11,647)
Tax (Expense) Benefit
Other Comprehensive Income, Net unrealized losses	1,868	4,068	4,657
Net of Tax
Other comprehensive loss	(4,229)	(6,108)	(6,990)
Available-for-sale securities:
Before-Tax Amount
Change in net unrealized gains (losses) during the period	(4,442)	(7,689)	(8,241)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	407	(926)	(1,108)
Other Comprehensive Income, Net unrealized losses	(4,035)	(8,615)	(9,349)
Tax (Expense) Benefit
Change in net unrealized gain during the period	1,761	3,074	3,295
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	(163)	370	443
Other Comprehensive Income, Net unrealized losses	1,598	3,444	3,738
Net of Tax
Change in net unrealized gain during the period	(2,681)	(4,615)	(4,946)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	244	(556)	(665)
Other comprehensive loss	(2,437)	(5,171)	(5,611)
Plan actuarial loss
Before-Tax Amount
Change in net unrealized gains (losses) during the period	(4,549)	(2,787)	1,846
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	1,508	1,338	(6,362)
Tax (Expense) Benefit
Change in net unrealized gain during the period	1,115	1,114	(738)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	(603)	(535)	2,544
Net of Tax
Change in net unrealized gain during the period	(3,434)	(1,673)	1,108
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	905	803	(3,818)
Net retirement plan prior service (cost) credit
Before-Tax Amount
Change in net unrealized gains (losses) during the period	964	(188)	2,218
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	15	76
Tax (Expense) Benefit
Change in net unrealized gain during the period	(236)	75	(887)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	(6)	(30)
Net of Tax
Change in net unrealized gain during the period	728	(113)	1,331
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	9	46
Employee benefit plans
Before-Tax Amount
Reclassification adjustment for net realized loss on sale included in available-for-sale securities			(2,298)
Other Comprehensive Income, Net unrealized losses	(2,062)	(1,561)
Tax (Expense) Benefit
Reclassification adjustment for net realized loss on sale included in available-for-sale securities			919
Other Comprehensive Income, Net unrealized losses	270	624
Net of Tax
Reclassification adjustment for net realized loss on sale included in available-for-sale securities			$ (1,379)
Other comprehensive loss	$ (1,792)	$ (937)