Other Comprehensive Income (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (265,721)	$ (298,386)	$ (265,406)	$ (354,998)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	1,782	(2,016)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	(10,029)	49,339
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	7,932	9,289
Other Comprehensive Income (Loss), Net of Tax	(315)	56,612
Accumulated Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(45,234)	(54,023)	(31,340)	(115,322)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	(13,894)	61,299
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0
Other Comprehensive Income (Loss), Net of Tax	(13,894)	61,299
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	1,383	(2,424)	(1,081)	(2,232)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	1,782	(2,016)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	682	1,824
Other Comprehensive Income (Loss), Net of Tax	2,464	(192)
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(221,870)	(241,939)	$ (232,985)	$ (237,444)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	3,865	(11,960)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	7,250	7,465
Other Comprehensive Income (Loss), Net of Tax	$ 11,115	$ (4,495)