Investment in American Transmission Company - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) member vote	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 29, 2015
Changes to investment in ATC
Equity in earnings of transmission affiliate	$ 146.5	$ 96.1	$ 66.0
Capital contributions	$ 42.3	8.7	13.1
ATC
Investment in ATC
Equity interest in ATC	60.00%			26.20%
Number of representatives on ATC's board of directors | member	1
Total number of members serving on ATC's board of directors | member	10
Number of votes that can be placed by each member on ATC's board of directors | vote	1
Number of members on ATC's board of directors with more than 10% voting control | member	0
Maximum voting control of any member on ATC's board of directors	10.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 1,380.9	424.1	402.7
Equity in earnings of transmission affiliate	146.5	96.1	66.0
Capital contributions	42.3	8.7	13.1
Acquisition of Integrys's investment in ATC	(1.0)	541.5	0.0
Equity method goodwill from the acquisition of Integrys	10.4	395.8	0.0
Distributions	135.1	85.1	57.5
Other	0.1	0.2	0.2
Investment in ATC, balance at end of period	1,443.9	$ 1,380.9	$ 424.1
Dividends not received
Dividends Receivable	$ 35.2