Pension and Postretirement Benefits - Amounts Recognized in AOCI (Details) - Domestic Plans - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Plans | Funded Plans
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	$ 8,681	$ 9,090
Subtotal	8,681	9,090
Less tax effect	(3,292)	(3,447)
Total	5,389	5,643
Pension Plans | Unfunded Pension Plans
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	2,587	2,496
Subtotal	2,587	2,496
Less tax effect	(981)	(947)
Total	1,606	1,549
Postretirement Benefit Plans
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	2,784	2,710
Prior service credit	(351)	(1,598)
Subtotal	2,433	1,112
Less tax effect	(923)	(422)
Total	1,510	690
US Postretirement and Pension Plan
Amounts recognized in accumulated other comprehensive income
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	14,052	14,296
Prior service credit	(351)	(1,598)
Subtotal	13,701	12,698
Less tax effect	(5,196)	(4,816)
Total	$ 8,505	$ 7,882