Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	1 Months Ended	12 Months Ended
	Nov. 30, 2019	Dec. 31, 2019 USD ($) member vote	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Changes to investment in transmission affiliates
Equity in earnings of transmission affiliates		$ 127.6	$ 136.7	$ 154.3
Capital contributions		52.6	53.5	109.6
Transmission Affiliates
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period		1,665.3	1,553.4	1,443.9
Equity in earnings of transmission affiliates		127.6	136.7	154.3
Capital contributions		52.6	53.5	109.6
Distributions		124.7	78.2	154.2
Other			0.1	0.2
Investment in ATC, balance at end of period		$ 1,720.8	1,665.3	1,553.4
ATC
Investment in Transmission Affiliates
Equity interest in ATC		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
Maximum voting control of any member on the transmission affiliate's board of directors		10.00%
Liability for potential future refunds that ATC may be required to provide		$ 41.9
Approved return on equity (as a percent)	10.38%
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period		1,625.3	1,515.8	1,443.9
Equity in earnings of transmission affiliates		132.8	139.6	166.0
Capital contributions		51.3	48.2	60.3
Distributions		124.7	78.2	154.2
Other			0.1	0.2
Investment in ATC, balance at end of period		$ 1,684.7	1,625.3	1,515.8
Dividends not received
Dividends Receivable				39.9	$ 35.2
ATC HoldCo
Investment in Transmission Affiliates
Equity interest in ATC		75.00%
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
Maximum voting control of any member on the transmission affiliate's board of directors		10.00%
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period		$ 40.0	37.6	0.0
Equity in earnings of transmission affiliates		(5.2)	(2.9)	(11.7)
Capital contributions		1.3	5.3	49.3
Distributions		0.0	0.0	0.0
Other			0.0	0.0
Investment in ATC, balance at end of period		$ 36.1	$ 40.0	$ 37.6