Employee Benefits - Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory asset	$ 3,378.7	$ 3,274.7
Pension Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	24.2	24.0	$ 50.8
Interest cost	116.6	122.3	91.8
Expected return on plan assets	(182.1)	(187.4)	(208.0)
Plan settlement	4.0	1.3	6.2
Amortization of prior service cost (credit)	(0.1)	0.0	1.6
Amortization of net actuarial loss (gain)	59.5	33.0	75.3
Net periodic benefit cost (credit)	22.1	(6.8)	17.7
Pension Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory asset	24.9	6.0
OPEB Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	10.9	9.8	14.3
Interest cost	22.7	21.6	15.4
Expected return on plan assets	(52.7)	(53.0)	(68.9)
Plan settlement	0.0	0.0	0.0
Amortization of prior service cost (credit)	(13.5)	(14.8)	(15.9)
Amortization of net actuarial loss (gain)	(7.6)	(12.3)	(24.7)
Net periodic benefit cost (credit)	(40.2)	(48.7)	$ (79.8)
OPEB Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory asset	$ 38.2	$ 14.8