Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,125,476)	$ (1,246,050)	$ (1,226,831)	$ (1,314,779)
Effect of pension settlement			(31,811)
Balance at end of period	(1,196,077)	(1,235,729)	(1,196,077)	(1,235,729)
Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	98,870	124,181	117,028	129,253
Net (income) loss arising during the period			19,402
Effect of pension settlement			(31,811)
Amounts reclassified from AOCL to earnings	(2,874)	(2,535)	(8,623)	(7,607)
Balance at end of period	95,996	121,646	95,996	121,646
Other Post Retirement Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	10,184	1,516	10,331	(1,382)
Amounts reclassified from AOCL to earnings	(74)	1,448	(221)	4,346
Balance at end of period	10,110	2,964	10,110	2,964
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	60,372	20,490	20,000	1,560
Net (income) loss arising during the period	(8,920)	7,384	34,112	26,576
Amounts reclassified from AOCL to earnings	(2,412)	(406)	(5,072)	(668)
Balance at end of period	49,040	27,468	49,040	27,468
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	169,426	146,187	147,359	129,431
Balance at end of period	$ 155,146	$ 152,078	$ 155,146	$ 152,078