Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	$ 665,865
Unrealized gains (losses)	6,039
Amounts reclassified from AOCI	(140)
Ending Balance	688,462
Accumulated Other Comprehensive Income
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	20,367
Ending Balance	26,266
Derivative instruments
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	647
Unrealized gains (losses)	6,697
Amounts reclassified from AOCI	(140)
Ending Balance	7,204
Pension, Postretirement and Other Postemployment Benefits
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	19,720
Unrealized gains (losses)	0
Amounts reclassified from AOCI	0	$ 0
Ending Balance	19,720
Available-for-sale securities
Accumulated Other Comprenhensive Income (Loss) [Roll Forward]
Beginning Balance	0
Unrealized gains (losses)	(658)
Amounts reclassified from AOCI	0
Ending Balance	$ (658)