Capital Structure and Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 1,396,506
Other comprehensive gain (loss) before reclassifications	3,773
Amounts reclassed from accumulated other comprehensive loss	490
Ending Balance	1,358,504
Unrealized gain/(loss) on cash flow hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(76)
Other comprehensive gain (loss) before reclassifications	0
Amounts reclassed from accumulated other comprehensive loss	6
Ending Balance	(70)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(6)
Other comprehensive gain (loss) before reclassifications	(541)
Amounts reclassed from accumulated other comprehensive loss	0
Ending Balance	(547)
Pension and other post-retirement benefits liability
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(13,844)
Other comprehensive gain (loss) before reclassifications	4,314
Amounts reclassed from accumulated other comprehensive loss	484
Ending Balance	(9,046)
Total
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(13,926)
Ending Balance	$ (9,663)