Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
Dec. 31, 2021 USD ($) member vote	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Changes to investment in transmission affiliates
Equity in earnings of transmission affiliates	$ 158.1	$ 175.8	$ 127.6
Capital contributions	0.0	21.2	52.6
Transmission affiliates
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period	1,764.3	1,720.8	1,665.3
Equity in earnings of transmission affiliates	158.1	175.8	127.6
Capital contributions		21.2	52.6
Distributions	133.0	146.7	124.7
Return of capital		6.8
Investment in ATC, balance at end of period	$ 1,789.4	1,764.3	1,720.8
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
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period	$ 1,733.5	1,684.7	1,625.3
Equity in earnings of transmission affiliates	166.4	174.3	132.8
Capital contributions		21.2	51.3
Distributions	133.0	146.7	124.7
Return of capital		0.0
Investment in ATC, balance at end of period	1,766.9	1,733.5	1,684.7
Liability for potential future refunds that ATC may be required to provide	$ 39.1
Approved return on equity (as a percent)	10.52%
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 ATC, balance at beginning of period	$ 30.8	36.1	40.0
Equity in earnings of transmission affiliates	(8.3)	1.5	(5.2)
Capital contributions		0.0	1.3
Distributions	0.0	0.0	0.0
Return of capital		6.8
Investment in ATC, balance at end of period	$ 22.5	$ 30.8	$ 36.1

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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