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Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions | 12 Months Ended
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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

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