Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income [Line Items]
Beginning Balance	$ 25,714
Cumulative effect of accounting change for adoption of ASU			$ 0	$ 386	$ 2,210
Other comprehensive income (loss)	28,631	$ 19,899	120,366	36,675
Ending Balance	52,544		25,714
Unrealized Gain (Loss) on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income [Line Items]
Beginning Balance	1,801
Cumulative effect of accounting change for adoption of ASU			(1,801)
Other comprehensive income before reclassifications	0
Tax effect	0
Other comprehensive income before reclassifications, net of tax	0
Amounts reclassified out of AOCI	0
Tax effect of reclassification	0
Amounts reclassified out of AOCI, net of tax	0
Other comprehensive income (loss)	0
Ending Balance	0		1,801
Currency Translation Adjustment [Member]
Accumulated Other Comprehensive Income [Line Items]
Beginning Balance	92,954
Cumulative effect of accounting change for adoption of ASU			0
Other comprehensive income before reclassifications	27,024
Tax effect	0
Other comprehensive income before reclassifications, net of tax	27,024
Amounts reclassified out of AOCI	0
Tax effect of reclassification	0
Amounts reclassified out of AOCI, net of tax	0
Other comprehensive income (loss)	27,024
Ending Balance	119,978		92,954
Pension and Other Post-Retirement Actuarial Items [Member]
Accumulated Other Comprehensive Income [Line Items]
Beginning Balance	(69,041)
Cumulative effect of accounting change for adoption of ASU			0
Other comprehensive income before reclassifications	0
Tax effect	0
Other comprehensive income before reclassifications, net of tax	0
Amounts reclassified out of AOCI	2,296
Tax effect of reclassification	(689)
Amounts reclassified out of AOCI, net of tax	1,607
Other comprehensive income (loss)	1,607
Ending Balance	(67,434)		(69,041)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income [Line Items]
Beginning Balance	25,714
Cumulative effect of accounting change for adoption of ASU			(1,801)	0	$ 0
Other comprehensive income before reclassifications	27,024
Tax effect	0
Other comprehensive income before reclassifications, net of tax	27,024
Amounts reclassified out of AOCI	2,296
Tax effect of reclassification	(689)
Amounts reclassified out of AOCI, net of tax	1,607
Other comprehensive income (loss)	28,631		120,366	$ 36,675
Ending Balance	$ 52,544		$ 25,714