Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,526,133)	$ (1,304,953)
Effect of pension settlement	(10,553)	0
Balance at end of period	(1,712,198)	(1,477,549)
Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	49,579	79,025
Net (gain) loss arising during the period	4,587
Effect of pension settlement	(10,553)
Amounts reclassified from AOCI to earnings	(1,469)	(2,770)
Balance at end of period	42,144	76,255
Other Postretirement Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	1,790	14,783
Amounts reclassified from AOCI to earnings	(45)	(183)
Balance at end of period	1,745	14,600
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(8,131)	27,181
Net (gain) loss arising during the period	(41,243)	(61,875)
Amounts reclassified from AOCI to earnings	(2,033)	(2,232)
Balance at end of period	(51,407)	(36,926)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	43,238	120,989
Balance at end of period	$ (7,518)	$ 53,929