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Significant Accounting Policies - Reclassifications Out of Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Additional interest expense $ 28.1 $ 31.2 $ 55.6 $ 61.0
Tax benefit 62.3 61.4 78.1 61.6
Pension and Postretirement Benefit Plans        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Earnings from Continuing Operations 2.1 2.4 4.6 4.9
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Benefit Plans        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Prior service cost (credit) 0.1     (0.1)
Tax benefit (0.8) (0.8) (1.5) (1.6)
Earnings from Continuing Operations 2.1 2.4 4.6 4.9
Reclassification out of Accumulated Other Comprehensive Income | Pension and Postretirement Benefit Plans | Other Nonoperating Income, Net        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]        
Actuarial loss $ 2.8 $ 3.2 $ 6.1 $ 6.6