Employee Benefits - Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	$ 50.8	$ 54.3	$ 50.1
Interest cost	91.8	87.5	102.8
Expected return on plan assets	(208.0)	(200.9)	(190.3)
Plan settlement	6.2	3.9	17.9
Plan curtailment	0.0	0.0	0.0
Amortization of prior service cost (credit)	1.6	1.6	1.6
Amortization of net actuarial loss (gain)	75.3	109.4	102.6
Net periodic benefit cost (credit)	17.7	55.8	84.7
OPEB Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	14.3	15.7	15.2
Interest cost	15.4	14.5	18.6
Expected return on plan assets	(68.9)	(66.0)	(60.3)
Plan settlement	0.0	0.0	0.0
Plan curtailment	0.0	(6.4)	0.0
Amortization of prior service cost (credit)	(15.9)	(15.9)	(15.0)
Amortization of net actuarial loss (gain)	(24.7)	(24.4)	(22.4)
Net periodic benefit cost (credit)	$ (79.8)	$ (82.5)	$ (63.9)