Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) member vote	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Changes to investment in transmission affiliates
Equity in earnings of transmission affiliates	$ 136.7	$ 154.3	$ 146.5
Capital contributions	53.5	109.6	42.3
Transmission Affiliates
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period	1,553.4	1,443.9
Equity in earnings of transmission affiliates	136.7	154.3
Capital contributions	53.5	109.6
Distributions	78.2	154.2
Other	0.1	0.2
Investment in ATC, balance at end of period	$ 1,665.3	1,553.4	1,443.9
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	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
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	$ 1,515.8	1,443.9	1,380.9
Equity in earnings of transmission affiliates	139.6	166.0	146.5
Capital contributions	48.2	60.3	42.3
Acquisition of Integrys's investment in ATC			(1.0)
Equity method goodwill from the acquisition of Integrys			10.4
Distributions	78.2	154.2	135.1
Other	0.1	0.2	0.1
Investment in ATC, balance at end of period	1,625.3	1,515.8	1,443.9
Dividends not received
Dividends Receivable	$ 0.0	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	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
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	$ 37.6	0.0
Equity in earnings of transmission affiliates	(2.9)	(11.7)
Capital contributions	5.3	49.3
Distributions	0.0	0.0
Other	0.0	0.0
Investment in ATC, balance at end of period	$ 40.0	$ 37.6	$ 0.0