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Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period $ (1,611,789) $ (1,125,476) $ (1,304,953) $ (1,226,831)
Effect of pension settlement 0 0 0 (31,811)
Balance at end of period (1,660,106) (1,196,077) (1,660,106) (1,196,077)
Pension Plans [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period 73,592 98,870 79,025 117,028
Amounts arising during the period       19,402
Effect of pension settlement       (31,811)
Amounts reclassified from AOCL to earnings (2,664) (2,874) (8,097) (8,623)
Balance at end of period 70,928 95,996 70,928 95,996
Other Postretirement Benefit Plans [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period 14,418 10,184 14,783 10,331
Amounts reclassified from AOCL to earnings (182) (74) (547) (221)
Balance at end of period 14,236 10,110 14,236 10,110
Cash Flow Hedges [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period (5,763) 60,372 27,181 20,000
Amounts arising during the period (4,310) (8,920) (32,741) 34,112
Amounts reclassified from AOCL to earnings (2,326) (2,412) (6,839) (5,072)
Balance at end of period (12,399) 49,040 (12,399) 49,040
Accumulated Other Comprehensive Loss [Member]        
Accumulated Other Comprehensive Income Loss [Line Items]        
Balance at beginning of period 82,247 169,426 120,989 147,359
Balance at end of period $ 72,765 $ 155,146 $ 72,765 $ 155,146