Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	$ 640,536	$ 704,821
Ending Balance	283,561	640,536
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	5,689	43,296
Unrealized gains (losses)	(41,675)	(22,536)
Amounts reclassified from AOCI	285	(15,071)
Ending Balance	(35,701)	5,689
Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(2,564)	3,328
Unrealized gains (losses)	(3,076)	4,648
Amounts reclassified from AOCI	1,749	(10,540)
Ending Balance	(3,891)	(2,564)
Pension, postretirement and other post-employment benefits
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	8,273	40,311
Unrealized gains (losses)	(38,533)	(27,692)
Amounts reclassified from AOCI	(1,199)	(4,346)
Ending Balance	(31,459)	8,273
Available-for-sale securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning Balance	(20)	(343)
Unrealized gains (losses)	(66)	508
Amounts reclassified from AOCI	(265)	(185)
Ending Balance	$ (351)	$ (20)