Accumulated Other Comprehensive Loss (Components of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 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		7,778	$ 4,024
Amounts reclassified from accumulated other comprehensive (income) loss		43	30
Other comprehensive income		7,821	4,054
Balance, End of Period		799,023
AOCI, Pension and other postretirement benefit plans, tax		9,549	9,144	$ 9,563	$ 9,160
AOCI, Cumulative changes in net gain (loss) from cash flow hedges, tax		225	(65)	$ (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		7,000	4,138
Amounts reclassified from accumulated other comprehensive (income) loss		0	0
Other comprehensive income		7,000	4,138
Balance, End of Period		(10,983)	(42,308)
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
Amounts reclassified from accumulated other comprehensive (income) loss		43	30
Other comprehensive income	[1]	43	30
Balance, End of Period	[1]	(47,155)	(45,786)
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		778	(114)
Amounts reclassified from accumulated other comprehensive (income) loss		0	0
Other comprehensive income	[2]	778	(114)
Balance, End of Period	[2]	804	(114)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Balance, Beginning of Period		(65,155)	(92,262)
Balance, End of Period		$ (57,334)	$ (88,208)

[1] Net of taxes of $9,549 and $9,563 as of March 31, 2018 and December 31, 2017, respectively. Net of taxes of $9,144 and $9,160 as of March 31, 2017 and December 31, 2016, respectively.

[2] Net of taxes of $225 and $15 as of March 31, 2018 and December 31, 2017, respectively. Net of taxes of $65 and $0 as of March 31, 2017 and December 31, 2016, respectively.