Investment in American Transmission Company - Changes to Investment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Jun. 29, 2015
Changes to investment in ATC
Investment in ATC, balance at beginning of period			$ 1,380.9
Add: Earnings from equity method investment	$ 30.9	$ 14.3	69.4	$ 30.4
Investment in ATC, balance at end of period	$ 1,425.0		$ 1,425.0
ATC
Investment in ATC
Equity method goodwill from the acquisition of Integrys					$ 395.8
Ownership interest in ATC (as a percent)	60.00%		60.00%			26.20%
Changes to investment in ATC
Investment in ATC, balance at beginning of period	$ 1,422.5	431.1	$ 1,380.9	424.1
Add: Earnings from equity method investment	30.9	14.3	69.4	30.4
Add: Capital contributions	3.1	1.2	12.1	2.5
Add: Acquisition of Integrys's investment in ATC	(1.0)	552.0	(1.0)	552.0
Add: Adjustment to equity method goodwill	1.1	0.0	10.4	0.0
Less: Distributions received	31.6	10.8	46.7	21.2
Less: Other	0.0	0.0	0.1	0.0
Investment in ATC, balance at end of period	$ 1,425.0	$ 987.8	$ 1,425.0	$ 987.8

This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

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Investment in common stock of an equity method investee acquired as part of a business combination.

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