Employee Benefits - Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	$ 50.1	$ 47.0	$ 47.1
Interest cost	102.8	120.4	114.3
Expected return on plan assets	(190.3)	(193.3)	(196.5)
Plan settlement	17.9	11.5	1.0
Amortization of prior service cost (credit)	1.6	2.2	2.7
Amortization of net actuarial loss (gain)	102.6	77.3	94.0
Net periodic benefit cost (credit)	84.7	65.1	62.6
OPEB Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	15.2	16.3	23.7
Interest cost	18.6	25.7	29.9
Expected return on plan assets	(60.3)	(54.7)	(59.5)
Plan settlement	0.0	0.0	0.0
Amortization of prior service cost (credit)	(15.0)	(15.4)	(15.4)
Amortization of net actuarial loss (gain)	(22.4)	(6.6)	1.0
Net periodic benefit cost (credit)	$ (63.9)	$ (34.7)	$ (20.3)