Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 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)	$ 5,947	$ 29,661	(25,384)	$ 104,810	120,366	36,675
Ending Balance	(1,471)		(1,471)		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			(30,236)
Tax effect			0
Other comprehensive income before reclassifications, net of tax			(30,236)
Amounts reclassified out of AOCI			0
Tax effect of reclassification			0
Amounts reclassified out of AOCI, net of tax			0
Other comprehensive income (loss)			(30,236)
Ending Balance	62,718		62,718		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			6,734
Tax effect of reclassification			(1,882)
Amounts reclassified out of AOCI, net of tax			4,852
Other comprehensive income (loss)			4,852
Ending Balance	(64,189)		(64,189)		(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			(30,236)
Tax effect			0
Other comprehensive income before reclassifications, net of tax			(30,236)
Amounts reclassified out of AOCI			6,734
Tax effect of reclassification			(1,882)
Amounts reclassified out of AOCI, net of tax			4,852
Other comprehensive income (loss)			(25,384)		120,366	$ 36,675
Ending Balance	$ (1,471)		$ (1,471)		$ 25,714