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 ($)
Changes to investment in ATC
Earnings from equity method investment	$ 55.5	$ 47.8	$ 57.9
Capital contributions	$ 16.1	4.6	11.5
ATC
Investment in ATC
Ownership interest in ATC (as a percent)	23.00%
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 of ATC's board of directors	10.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 382.2	372.9	354.1
Earnings from equity method investment	55.5	47.8	57.9
Capital contributions	16.1	4.6	11.5
Distributions	51.7	42.9	50.5
Other	0.1	0.2	0.1
Investment in ATC, balance at end of period	402.0	$ 382.2	$ 372.9
Dividends not received
Dividends Receivable	$ 13.4