Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
Dec. 31, 2023 USD ($) vote member	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Changes to investment in transmission affiliates
Capital contributions	$ 63.7	$ 45.5	$ 0.0
Transmission affiliates
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,909.2	1,789.4	1,764.3
Earnings (Loss) from Equity Method Investment	177.5	194.7	158.1
Capital contributions	63.7	45.5
Distributions	144.5	120.4	133.0
Investment in transmission affiliates, balance at end of period	$ 2,005.9	1,909.2	1,789.4
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
Liability for potential future refunds that ATC may be required to provide			39.1	$ 39.1
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	$ 1,884.6	1,766.9	1,733.5
Earnings (Loss) from Equity Method Investment	175.1	192.6	166.4
Capital contributions	63.7	45.5
Distributions	142.6	120.4	133.0
Investment in transmission affiliates, balance at end of period	$ 1,980.8	1,884.6	1,766.9
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	$ 24.6	22.5	30.8
Earnings (Loss) from Equity Method Investment	2.4	2.1	(8.3)
Capital contributions	0.0	0.0
Distributions	1.9	0.0	0.0
Investment in transmission affiliates, balance at end of period	$ 25.1	$ 24.6	$ 22.5

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

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