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Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period $ (1,226,831) $ (1,314,779)
Effect of pension settlement (31,811)  
Balance at end of period (1,304,953) (1,226,831)
Pension Plans [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 117,028 129,253
Net loss (gain) arising during the period 5,306 (2,081)
Effect of pension settlement (31,811)  
Amounts reclassified from AOCL to earnings (11,498) (10,144)
Balance at end of period 79,025 117,028
Other Postretirement Benefit Plans [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 10,331 (1,382)
Net loss (gain) arising during the period 4,747 5,917
Amounts reclassified from AOCL to earnings (295) 5,796
Balance at end of period 14,783 10,331
Cash Flow Hedges [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 20,000 1,560
Net loss (gain) arising during the period 14,632 19,824
Amounts reclassified from AOCL to earnings (7,451) (1,384)
Balance at end of period 27,181 20,000
Accumulated Other Comprehensive Loss [Member]    
Accumulated Other Comprehensive Income Loss [Line Items]    
Balance at beginning of period 147,359 129,431
Balance at end of period $ 120,989 $ 147,359