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 liabilities	$ 1,636.7	$ 1,638.8
Regulatory assets	2,860.7	2,817.5
Pension Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	10.3	13.4	$ 14.4
Interest cost	47.1	32.6	31.1
Expected return on plan assets	(64.3)	(71.4)	(70.3)
Amortization of prior service credit	(0.1)	(0.1)	(0.1)
Amortization of net actuarial loss (gain)	9.2	29.8	41.9
Net periodic benefit cost (credit)	2.2	4.3	17.0
Pension Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory liabilities	1.2
OPEB Benefits
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Service cost	2.6	3.9	4.3
Interest cost	7.7	5.6	5.3
Expected return on plan assets	(13.4)	(17.7)	(16.9)
Amortization of prior service credit	(0.9)	(1.3)	(1.2)
Amortization of net actuarial loss (gain)	(8.7)	(12.1)	(10.9)
Net periodic benefit cost (credit)	(12.7)	$ (21.6)	$ (19.4)
OPEB Benefits | Pension and Other Postretirement Plans Cost
Components of net periodic benefit cost (credit) (including amounts capitalized to the balance sheets)
Regulatory assets	$ 5.3