Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,226,831)	$ (1,314,779)
Effect of pension settlement	(31,811)
Balance at end of period	(1,149,614)	(1,278,386)
Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	117,028	129,253
Net loss arising during the period	19,402
Effect of pension settlement	(31,811)
Amounts reclassified from AOCL to earnings	(3,095)	(2,575)
Balance at end of period	101,524	126,678
Other Post Retirement Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	10,331	(1,382)
Amounts reclassified from AOCL to earnings	(74)	1,409
Balance at end of period	10,257	27
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	20,000	1,560
Net loss arising during the period	39,363	8,594
Amounts reclassified from AOCL to earnings	(838)	(81)
Balance at end of period	58,525	10,073
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	147,359	129,431
Balance at end of period	$ 170,306	$ 136,778