Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 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)	$ (59,962)	$ 55,250	(31,331)	$ 75,149	120,366	36,675
Ending Balance	(7,418)		(7,418)		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		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			(34,513)
Tax effect			0
Other comprehensive income before reclassifications, net of tax			(34,513)
Amounts reclassified out of AOCI			0
Tax effect of reclassification			0
Amounts reclassified out of AOCI, net of tax			0
Other comprehensive income (loss)			(34,513)
Ending Balance	58,441		58,441		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			4,547
Tax effect of reclassification			(1,365)
Amounts reclassified out of AOCI, net of tax			3,182
Other comprehensive income (loss)			3,182
Ending Balance	(65,859)		(65,859)		(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			(34,513)
Tax effect			0
Other comprehensive income before reclassifications, net of tax			(34,513)
Amounts reclassified out of AOCI			4,547
Tax effect of reclassification			(1,365)
Amounts reclassified out of AOCI, net of tax			3,182
Other comprehensive income (loss)			(31,331)		120,366	$ 36,675
Ending Balance	$ (7,418)		$ (7,418)		$ 25,714