Accumulated Other Comprehensive Loss (Components of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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				(7,196)	$ 16,295
Actuarial net gain incurred in the fiscal year					334
Amounts reclassified from accumulated other comprehensive (income) loss				87	143
Other comprehensive income (loss)		$ (14,930)	$ 12,717	(7,109)	16,772
Balance, End of Period		804,785		804,785
AOCI, Pension and other postretirement benefit plans, tax		9,536	8,961	9,536	8,961	$ 9,563	$ 9,160
AOCI, Cumulative changes in net gain (loss) from cash flow hedges, tax		316	(187)	316	(187)	$ (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				(8,293)	16,730
Actuarial net gain incurred in the fiscal year					0
Amounts reclassified from accumulated other comprehensive (income) loss				0	0
Other comprehensive income (loss)				(8,293)	16,730
Balance, End of Period		(26,276)	(29,716)	(26,276)	(29,716)
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					334
Amounts reclassified from accumulated other comprehensive (income) loss				87	36
Other comprehensive income (loss)	[1]			87	370
Balance, End of Period	[1]	(47,111)	(45,446)	(47,111)	(45,446)
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				1,097	(435)
Actuarial net gain incurred in the fiscal year					0
Amounts reclassified from accumulated other comprehensive (income) loss				0	107
Other comprehensive income (loss)	[2]			1,097	(328)
Balance, End of Period	[2]	1,123	(328)	1,123	(328)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period				(65,155)	(92,262)
Other comprehensive income (loss)				(7,109)
Balance, End of Period		$ (72,264)	$ (75,490)	$ (72,264)	$ (75,490)

[1] Net of taxes of $9,536 and $9,563 as of June 30, 2018 and December 31, 2017, respectively. Net of taxes of $8,961 and $9,160 as of June 30, 2017 and December 31, 2016, respectively.

[2] Net of taxes of $316 and $15 as of June 30, 2018 and December 31, 2017, respectively. Net of taxes of $187 and $0 as of June 30, 2017 and December 31, 2016, respectively.