Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ (244,761)	$ (187,533)	$ (250,154)	$ (196,600)
Other comprehensive income before reclassifications	1,695	3,527	(2,456)	6,322
Amounts reclassified from accumulated other comprehensive income (net of tax)	4,773	3,136	14,317	9,408
Net current-period other comprehensive income	6,468	6,663	11,861	15,730
Ending balance	(238,293)	(180,870)	(238,293)	(180,870)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(1,654)	(1,997)	(4,964)	(5,990)
Foreign Currency
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(8,753)	(7,548)	(4,602)	(10,343)
Other comprehensive income before reclassifications	1,695	3,527	(2,456)	6,322
Amounts reclassified from accumulated other comprehensive income (net of tax)	0	0	0	0
Net current-period other comprehensive income	1,695	3,527	(2,456)	6,322
Ending balance	(7,058)	(4,021)	(7,058)	(4,021)
Pension and Other Postretirement Benefits
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(236,008)	(179,985)	(245,552)	(186,257)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (net of tax)	4,773	3,136	14,317	9,408
Net current-period other comprehensive income	4,773	3,136	14,317	9,408
Ending balance	$ (231,235)	$ (176,849)	$ (231,235)	$ (176,849)