Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	$ 665,865
Unrealized gains (losses)	(689)
Amounts reclassified from AOCI	(1,875)
Ending Balance	640,948
Accumulated Other Comprehensive Income
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	20,367
Ending Balance	17,803
Derivative instruments
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	647
Unrealized gains (losses)	(5,248)
Amounts reclassified from AOCI	(488)
Ending Balance	(5,089)
Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	19,720
Unrealized gains (losses)	5,024
Amounts reclassified from AOCI	(1,371)
Ending Balance	23,373
Available-for-sale securities
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	0
Unrealized gains (losses)	(465)
Amounts reclassified from AOCI	(16)
Ending Balance	$ (481)