Variable Interest Entities - Investment in Transmission Affiliates (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
ATC
Variable interest entities
Ownership interest (as a percent)	60.00%
Equity investment	$ 1,980.8	$ 1,884.6	$ 1,766.9	$ 1,733.5
ATC Holdco
Variable interest entities
Ownership interest (as a percent)	75.00%
Equity investment	$ 25.1	$ 24.6	$ 22.5	$ 30.8