Employee Benefits - Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	$ 54.3	$ 50.1	$ 47.0
Interest cost	87.5	102.8	120.4
Expected return on plan assets	(200.9)	(190.3)	(193.3)
Plan settlement	3.9	17.9	11.5
Plan curtailment	0.0	0.0	0.0
Amortization of prior service cost (credit)	1.6	1.6	2.2
Amortization of net actuarial loss (gain)	109.4	102.6	77.3
Net periodic benefit cost (credit)	55.8	84.7	65.1
OPEB Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	15.7	15.2	16.3
Interest cost	14.5	18.6	25.7
Expected return on plan assets	(66.0)	(60.3)	(54.7)
Plan settlement	0.0	0.0	0.0
Plan curtailment	(6.4)	0.0	0.0
Amortization of prior service cost (credit)	(15.9)	(15.0)	(15.4)
Amortization of net actuarial loss (gain)	(24.4)	(22.4)	(6.6)
Net periodic benefit cost (credit)	$ (82.5)	$ (63.9)	$ (34.7)