Pension and Other Postretirement Benefit Costs - Amounts Recognized in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Reclassification of Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (6,678)	$ (6,825)	$ (6,951)	$ (7,162)
Amounts reclassified from AOCL:
Amortization of actuarial losses	248	371	698	962
Total reclassifications before tax	248	(1,231)	698	(640)
Tax (benefit) expense	(98)	486	(275)	232
Total reclassifications for the period	150	(745)	423	(408)
Ending balance	(6,528)	(7,570)	(6,528)	(7,570)
Non-Qualified Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Reclassification of Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Amounts reclassified to AOCL	0	(1,795)	0	(1,795)
Amounts reclassified from AOCL:
Loss from plan settlement	$ 0	$ 193	$ 0	$ 193