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Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period $ (1,304,953) $ (1,226,831)
Effect of pension settlement   (31,811)
Balance at end of period (1,477,549) (1,149,614)
Pension Plans [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 79,025 117,028
Amounts arising during the period   19,402
Effect of pension settlement   (31,811)
Amounts reclassified from AOCL to earnings (2,770) (3,095)
Balance at end of period 76,255 101,524
Other Postretirement Benefit Plans [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 14,783 10,331
Amounts reclassified from AOCL to earnings (183) (74)
Balance at end of period 14,600 10,257
Cash Flow Hedges [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 27,181 20,000
Amounts arising during the period (61,875) 39,363
Amounts reclassified from AOCL to earnings (2,232) (838)
Balance at end of period (36,926) 58,525
Accumulated Other Comprehensive Loss [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 120,989 147,359
Balance at end of period $ 53,929 $ 170,306