Other Comprehensive Income (Loss) - Tax Effect Allocated to Each Component of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Before-Tax Amount
Other Comprehensive Income, Net unrealized losses	$ (15,813)	$ (6,097)	$ (10,176)
Tax (Expense) Benefit
Other Comprehensive Income, Net unrealized losses	3,879	1,868	4,068
Net of Tax
Other comprehensive loss	(11,934)	(4,229)	(6,108)
Available-for-sale securities:
Before-Tax Amount
Change in net unrealized gains (losses) during the period	(14,550)	(4,442)	(7,689)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	440	407	(926)
Other Comprehensive Income, Net unrealized losses	(14,110)	(4,035)	(8,615)
Tax (Expense) Benefit
Change in net unrealized loss during the period	3,569	1,761	3,074
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	(108)	(163)	370
Other Comprehensive Income, Net unrealized losses	3,461	1,598	3,444
Net of Tax
Change in net unrealized loss during the period	(10,981)	(2,681)	(4,615)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	332	244	(556)
Other comprehensive loss	(10,649)	(2,437)	(5,171)
Plan actuarial loss
Before-Tax Amount
Change in net unrealized gains (losses) during the period	(3,437)	(4,549)	(2,787)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	1,719	1,508	1,338
Tax (Expense) Benefit
Change in net unrealized loss during the period	843	1,115	1,114
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	(421)	(603)	(535)
Net of Tax
Change in net unrealized loss during the period	(2,594)	(3,434)	(1,673)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	1,298	905	803
Net retirement plan prior service (cost) credit
Before-Tax Amount
Change in net unrealized gains (losses) during the period		964	(188)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	15	15	76
Tax (Expense) Benefit
Change in net unrealized loss during the period		(236)	75
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	(4)	(6)	(30)
Net of Tax
Change in net unrealized loss during the period		728	(113)
Reclassification adjustment for net realized loss on sale included in available-for-sale securities	11	9	46
Employee benefit plans
Before-Tax Amount
Other Comprehensive Income, Net unrealized losses	(1,703)	(2,062)	(1,561)
Tax (Expense) Benefit
Other Comprehensive Income, Net unrealized losses	418	270	624
Net of Tax
Other comprehensive loss	$ (1,285)	$ (1,792)	$ (937)