Employee Benefits - Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory asset	$ 3,274.7	$ 3,306.9
Pension Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	24.0	50.8	$ 54.3
Interest cost	122.3	91.8	87.5
Expected return on plan assets	(187.4)	(208.0)	(200.9)
Plan settlement	1.3	6.2	3.9
Plan curtailment	0.0	0.0	0.0
Amortization of prior service cost (credit)	0.0	1.6	1.6
Amortization of net actuarial loss (gain)	33.0	75.3	109.4
Net periodic benefit cost (credit)	(6.8)	17.7	55.8
Pension Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory asset	6.0
OPEB Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	9.8	14.3	15.7
Interest cost	21.6	15.4	14.5
Expected return on plan assets	(53.0)	(68.9)	(66.0)
Plan settlement	0.0	0.0	0.0
Plan curtailment	0.0	0.0	(6.4)
Amortization of prior service cost (credit)	(14.8)	(15.9)	(15.9)
Amortization of net actuarial loss (gain)	(12.3)	(24.7)	(24.4)
Net periodic benefit cost (credit)	(48.7)	$ (79.8)	$ (82.5)
OPEB Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory asset	$ 14.8