Investment in Transmission Affiliates - Changes to Investment (Details) $ in Millions	12 Months Ended
Dec. 31, 2024 USD ($) vote member	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 2,081.6	$ 1,981.9
Capital contributions	45.5	63.7	$ 45.5
Investment in transmission affiliates, balance at end of period	2,191.6	2,081.6	1,981.9
Transmission affiliates
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	2,005.9	1,909.2	1,789.4
Earnings (loss) from equity method investment	207.5	177.5	194.7
Capital contributions	45.5	63.7	45.5
Distributions	150.1	144.5	120.4
Other	0.1
Investment in transmission affiliates, balance at end of period	$ 2,108.9	2,005.9	1,909.2
ATC
Investment in transmission affiliates
Equity method investment, ownership interest (as a percent)	60.00%
Total number of members serving on the transmission affiliate's board of directors \| member	10
Number of representatives on the transmission affiliate's board of directors \| member	1
Number of votes that can be placed by each member on the transmission affiliate's board of directors \| vote	1
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 1,980.8	1,884.6	1,766.9
Earnings (loss) from equity method investment	205.4	175.1	192.6
Capital contributions	45.5	63.7	45.5
Distributions	146.7	142.6	120.4
Other	0.1
Investment in transmission affiliates, balance at end of period	$ 2,085.1	1,980.8	1,884.6
ATC Holdco
Investment in transmission affiliates
Equity method investment, ownership interest (as a percent)	75.00%
Total number of members serving on the transmission affiliate's board of directors \| member	4
Number of representatives on the transmission affiliate's board of directors \| member	1
Number of votes that can be placed by each member on the transmission affiliate's board of directors \| vote	1
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 25.1	24.6	22.5
Earnings (loss) from equity method investment	2.1	2.4	2.1
Capital contributions	0.0	0.0	0.0
Distributions	3.4	1.9	0.0
Other	0.0
Investment in transmission affiliates, balance at end of period	$ 23.8	$ 25.1	$ 24.6

Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

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For the members on an equity method investee's board of directors, the number of votes that each member can place.

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Other items not separately disclosed that impact investments accounted for under the equity method of accounting.

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The number of representatives the entity has on an equity method investee's board of directors.

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