Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,712,198)	$ (1,477,549)	$ (1,526,133)	$ (1,304,953)
Effect of pension settlement	0	0	(10,553)	0
Balance at end of period	(1,843,551)	(1,611,789)	(1,843,551)	(1,611,789)
Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	42,144	76,255	49,579	79,025
Net (gain) loss arising during the period	0	0	4,587	0
Effect of pension settlement	0	0	(10,553)	0
Amounts reclassified from AOCI to earnings	(870)	(2,663)	(2,339)	(5,433)
Balance at end of period	41,274	73,592	41,274	73,592
Other Postretirement Benefit Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	1,745	14,600	1,790	14,783
Amounts reclassified from AOCI to earnings	(45)	(182)	(90)	(365)
Balance at end of period	1,700	14,418	1,700	14,418
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(51,407)	(36,926)	(8,131)	27,181
Net (gain) loss arising during the period	(41,118)	33,444	(82,361)	(28,431)
Amounts reclassified from AOCI to earnings	(1,411)	(2,281)	(3,444)	(4,513)
Balance at end of period	(93,936)	(5,763)	(93,936)	(5,763)
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(7,518)	53,929	43,238	120,989
Balance at end of period	$ (50,962)	$ 82,247	$ (50,962)	$ 82,247