Accumulated Other Comprehensive Income (Loss) (Reclassifications out of AOCI) (Details 3) - USD ($) $ in Thousands		3 Months Ended					12 Months Ended
		Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income (expense), net		$ (9,187)	$ 2,640	$ (1,963)	$ (4,069)	$ (849)	$ (4,241)	$ 12,642
Interest expense		8,071				8,129
Provision for Income Taxes		13,600				2,700
Net of Tax		(42,965)				(21,086)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net of Tax		(941)				(997)
Foreign Currency Translation Adjustment [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax		0
Unrealized Gain on Available-for-Sale Securities [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax		0
Pension and Other Postretirement Plans [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax		(1,291)				(1,703)
Provision for Income Tax		348				681
Reclassifications, net of tax		(943)				(1,022)
Net Prior Service Cost [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1]	76				120
Provision for Income Tax		(21)				(48)
Reclassifications, net of tax		55				72
Net Actuarial (Gain) Loss [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, before tax	[1]	(1,367)				(1,823)
Provision for Income Tax		369				729
Reclassifications, net of tax		(998)				(1,094)
Cash Flow Hedge [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassifications, net of tax		2
Cash Flow Hedge [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense		3				31
Provision for Income Taxes		(1)				(6)
Net of Tax		$ 2				$ 25

[1] These accumulated other comprehensive income components are components of net periodic pension and postretirement plan cost (see Note 12) and are included in non-operating pension and postretirement benefit income in the Company's Condensed Consolidated Statements of Operations.