Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
Dec. 31, 2022 USD ($) member vote	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Changes to investment in transmission affiliates
Add: Capital contributions	$ 45.5	$ 0.0	$ 21.2
Transmission affiliates
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,789.4	1,764.3	1,720.8
Add: Earnings from equity method investment	194.7	158.1	175.8
Add: Capital contributions	45.5		21.2
Less: Distributions	120.4	133.0	146.7
Less: Return of capital			6.8
Investment in transmission affiliates, balance at end of period	$ 1,909.2	1,789.4	1,764.3
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	11
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
Changes to investment in transmission affiliates
Investment in transmission affiliates, balance at beginning of period	1,766.9	1,733.5	1,684.7
Add: Earnings from equity method investment	192.6	166.4	174.3
Add: Capital contributions	45.5		21.2
Less: Distributions	120.4	133.0	146.7
Less: Return of capital			0.0
Investment in transmission affiliates, balance at end of period	$ 1,884.6	1,766.9	1,733.5
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	$ 22.5	30.8	36.1
Add: Earnings from equity method investment	2.1	(8.3)	1.5
Add: Capital contributions	0.0		0.0
Less: Distributions	0.0	0.0	0.0
Less: Return of capital			6.8
Investment in transmission affiliates, balance at end of period	$ 24.6	$ 22.5	$ 30.8

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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