Other Comprehensive Income - Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Before Tax Amount
Change in net unrealized loss during period	$ (6,545)	$ (19,379)	$ 3,151
Reclassification adjustment for net gains included in net income	(718)	(330)	(137)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	1,800	1,900	1,600
Change in net unrealized loss on derivatives	(1,100)	(200)	200
Reclassification adjustment for derivative net (gains) losses included in net income	1,132	0	0
Change in net actuarial loss (gain)	(1,200)	4,000	100
Total other comprehensive loss	(6,600)	(14,000)	4,800
Tax Expense (Benefit)
Change in net unrealized loss during period	(2,700)	(7,600)	1,200
Reclassification adjustment for net gains included in net income	(300)	(100)	(100)
Unamortized premium on available-to-sale securities transferred into held-for-maturity	700	700	600
Change in net unrealized gain on derivatives	(400)	(100)	100
Reclassification adjustment for derivative net (gains) losses included in net income	400
Change in net actuarial loss	(500)	1,600	100
Total other comprehensive income (loss)	(2,800)	(5,500)	1,900
Net of Tax Amount
Change in net unrealized loss during period	(3,800)	(11,800)	1,900
Reclassification adjustment for net gains included in net income	(400)	(200)	(100)
Change in unamortized gain on available-for-sale securities transferred into held-to-maturity	1,100	1,200	1,000
Change in net unrealized gain on derivatives	(700)	(100)	100
Reclassification adjustment for derivative net (gains) losses included in net income	700
Change in net actuarial loss	(700)	2,400	0
Other comprehensive income (loss), net of tax	$ (3,835)	$ (8,534)	$ 2,940