Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	$ 665,865
Unrealized gains (losses)	(1,725)
Amounts reclassified from AOCI	2,063
Ending Balance	687,140
Accumulated Other Comprehensive Income
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	20,367
Ending Balance	20,705
Derivative instruments
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	647
Unrealized gains (losses)	(9,743)
Amounts reclassified from AOCI	4,071
Ending Balance	(5,025)
Pension, postretirement and other post-employment benefits
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	19,720
Unrealized gains (losses)	8,373
Amounts reclassified from AOCI	(1,984)
Ending Balance	26,109
Available-for-sale securities
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	0
Unrealized gains (losses)	(355)
Amounts reclassified from AOCI	(24)
Ending Balance	$ (379)