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Employee Benefits - Net Periodic Benefit Cost (Credit) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Benefits      
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)      
Service cost $ 47.0 $ 47.1 $ 44.6
Interest cost 120.4 114.3 121.8
Expected return on plan assets (193.3) (196.5) (195.7)
Plan settlement 11.5 1.0 9.0
Amortization of prior service cost (credit) 2.2 2.7 2.9
Amortization of net actuarial loss 77.3 94.0 86.1
Net periodic benefit cost (credit) 65.1 62.6 68.7
OPEB Benefits      
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)      
Service cost 16.3 23.7 24.1
Interest cost 25.7 29.9 32.9
Expected return on plan assets (54.7) (59.5) (55.5)
Plan settlement 0.0 0.0 0.0
Amortization of prior service cost (credit) (15.4) (15.4) (12.3)
Amortization of net actuarial loss (6.6) 1.0 3.1
Net periodic benefit cost (credit) $ (34.7) $ (20.3) $ (7.7)