Retirement Plans - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Total changes	$ 0	$ (536)	$ (407)
Pension plan
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	556	(482)
Recognition of net actuarial gain or (loss)	(213)	(849)
Recognized prior service cost	29	29
Total changes	372	(1,302)
Changes, net of taxes	188	(781)
SERP
Other changes in plan assets and benefit obligations recognized in other comprehensive income:
Net loss (gain)	129	726
Recognition of net actuarial gain or (loss)	(343)	(337)
Total changes	(214)	389
Changes, net of taxes	$ 638	$ 233