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Investment in Transmission Affiliates - Changes to Investment in ATC (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) vote member	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Changes to investment in transmission affiliates
Investment in ATC, balance at beginning of period	$ 1,443.9
Equity in earnings of transmission affiliates	154.3	$ 146.5	$ 96.1
Capital contributions	109.6	42.3	8.7
Distributions	154.2
Other	0.2
Investment in ATC, balance at end of period	$ 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	$ 1,443.9	1,380.9	424.1
Equity in earnings of transmission affiliates	166.0	146.5	96.1
Capital contributions	60.3	42.3	8.7
Acquisition of Integrys's investment in ATC		(1.0)	541.5
Equity method goodwill from the acquisition of Integrys		10.4	395.8
Distributions	154.2	135.1	85.1
Other	0.2	0.1	0.2
Investment in ATC, balance at end of period	1,515.8	1,443.9	$ 1,380.9
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	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	$ 0.0
Equity in earnings of transmission affiliates	(11.7)
Capital contributions	49.3
Distributions	0.0
Other	0.0
Investment in ATC, balance at end of period	$ 37.6	$ 0.0

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Carrying amount as of the balance sheet date of dividends declared but not received.

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

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