Pension and Other Postretirement Benefit Costs - Amounts Recognized in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Plan, Reclassification of Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (6,815)	$ (6,968)	$ (6,951)	$ (7,162)
Amounts reclassified from AOCL:
Amortization of actuarial losses	225	269	450	590
Total reclassifications before tax	225	269	450	590
Tax (benefit) expense	(88)	(126)	(177)	(253)
Total reclassifications for the period	137	143	273	337
Ending balance	(6,678)	(6,825)	(6,678)	(6,825)
Non-Qualified Pension Plans, Defined Benefit
Defined Benefit Plan, Reclassification of Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Amounts reclassified to AOCL	$ 0	$ 0	$ 0	$ 0