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Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period $ (1,477,549) $ (1,149,614) $ (1,304,953) $ (1,226,831)
Effect of pension settlement       (31,811)
Balance at end of period (1,611,789) (1,125,476) (1,611,789) (1,125,476)
Pension Plans [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period 76,255 101,524 79,025 117,028
Amounts arising during the period       19,402
Effect of pension settlement       (31,811)
Amounts reclassified from AOCL to earnings (2,663) (2,654) (5,433) (5,749)
Balance at end of period 73,592 98,870 73,592 98,870
Other Postretirement Benefit Plans [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period 14,600 10,257 14,783 10,331
Amounts reclassified from AOCL to earnings (182) (73) (365) (147)
Balance at end of period 14,418 10,184 14,418 10,184
Cash Flow Hedges [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period (36,926) 58,525 27,181 20,000
Amounts arising during the period 33,444 3,669 (28,431) 43,032
Amounts reclassified from AOCL to earnings (2,281) (1,822) (4,513) (2,660)
Balance at end of period (5,763) 60,372 (5,763) 60,372
Accumulated Other Comprehensive Loss [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period 53,929 170,306 120,989 147,359
Balance at end of period $ 82,247 $ 169,426 $ 82,247 $ 169,426