Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) vote member	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Changes to investment in transmission affiliates
Equity in earnings of transmission affiliates	$ 175.8	$ 127.6	$ 136.7
Capital contributions	21.2	52.6	53.5
Transmission affiliates
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period	1,720.8	1,665.3	1,553.4
Equity in earnings of transmission affiliates	175.8	127.6	136.7
Capital contributions	21.2	52.6	53.5
Distributions	146.7	124.7	78.2
Return of capital	6.8
Other			0.1
Investment in ATC, balance at end of period	$ 1,764.3	1,720.8	1,665.3
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	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 ATC, balance at beginning of period	$ 1,684.7	1,625.3	1,515.8
Equity in earnings of transmission affiliates	174.3	132.8	139.6
Capital contributions	21.2	51.3	48.2
Distributions	146.7	124.7	78.2
Return of capital	0.0
Other			0.1
Investment in ATC, balance at end of period	1,733.5	1,684.7	1,625.3
Dividends not received
Dividends receivable				$ 39.9
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	$ 36.1	40.0	37.6
Equity in earnings of transmission affiliates	1.5	(5.2)	(2.9)
Capital contributions	0.0	1.3	5.3
Distributions	0.0	0.0	0.0
Return of capital	6.8
Other			0.0
Investment in ATC, balance at end of period	$ 30.8	$ 36.1	$ 40.0