Retirement Plans - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Total changes	$ 0	$ 0	$ (536)
Pension plan
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	(355)	556
Recognition of net actuarial gain or (loss)	(137)	(213)
Recognized prior service cost	61	29
Total changes	(431)	372
Changes, net of taxes	(259)	188
SERP
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	(101)	129
Recognition of net actuarial gain or (loss)	(212)	(343)
Total changes	(313)	(214)
Changes, net of taxes	$ 188	$ 638