Pension and Other Postretirement Benefit Costs - Amounts Recognized in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Beginning balance	$ (8,438)	$ (6,951)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	209	225
Total reclassifications before tax	209	225
Tax (benefit) expense	(55)	(89)
Total reclassifications for the period	154	136
Ending balance	$ (8,284)	$ (6,815)