Accumulated Other Comprehensive Loss (Components of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period				$ 766,573
Other comprehensive income (loss) before reclassifications				(8,593)	$ 22,649
Actuarial net gain incurred in the fiscal year					35
Amounts reclassified from accumulated other comprehensive (income) loss				131	258
Other comprehensive income (loss)		$ (1,353)	$ 6,171	(8,462)	22,942
Balance, End of Period		826,383		826,383
AOCI, Pension and other postretirement benefit plans, tax		9,523	9,122	9,523	9,122	$ 9,563	$ 9,160
AOCI, Cumulative changes in net gain (loss) from cash flow hedges, tax		375	151	375	151	$ 15	$ 0
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period				(17,983)	(46,446)
Other comprehensive income (loss) before reclassifications				(9,901)	23,136
Actuarial net gain incurred in the fiscal year					0
Amounts reclassified from accumulated other comprehensive (income) loss				0	0
Other comprehensive income (loss)				(9,901)	23,136
Balance, End of Period		(27,884)	(23,310)	(27,884)	(23,310)
Funded status of pension plans and other postretirement benefits
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period				(47,198)	(45,816)
Other comprehensive income (loss) before reclassifications				0	0
Actuarial net gain incurred in the fiscal year					35
Amounts reclassified from accumulated other comprehensive (income) loss				131	36
Other comprehensive income (loss)	[1]			131	71
Balance, End of Period	[1]	(47,067)	(45,745)	(47,067)	(45,745)
Unrealized gain (loss) on derivative instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period				26	0
Other comprehensive income (loss) before reclassifications				1,308	(487)
Actuarial net gain incurred in the fiscal year					0
Amounts reclassified from accumulated other comprehensive (income) loss				0	222
Other comprehensive income (loss)	[2]			1,308	(265)
Balance, End of Period	[2]	1,334	(265)	1,334	(265)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period				(65,155)	(92,262)
Other comprehensive income (loss)				(8,462)
Balance, End of Period		$ (73,617)	$ (69,320)	$ (73,617)	$ (69,320)

[1] Net of taxes of $9,523 and $9,563 as of September 30, 2018 and December 31, 2017, respectively. Net of taxes of $9,122 and $9,160 as of September 30, 2017 and December 31, 2016, respectively.

[2] Net of taxes of $375 and $15 as of September 30, 2018 and December 31, 2017, respectively. Net of taxes of $151 and $0 as of September 30, 2017 and December 31, 2016, respectively.