INVESTMENT IN AMERICAN TRANSMISSION COMPANY - CHANGES TO INVESTMENT (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Changes to investment in ATC
Investment in ATC, balance at beginning of period			$ 402.0
Add: Earnings from equity method investment	$ 0.0	$ 14.6	0.0	$ 40.7
Add: Capital contributions			0.0	10.4
Investment in ATC, balance at end of period	0.0		0.0
ATC
Investment in ATC
Ownership interest in ATC (as a percent)					23.00%
Changes to investment in ATC
Investment in ATC, balance at beginning of period		394.8	402.0	382.2
Less: Transfer of ownership interest		0.0	402.0	0.0
Add: Earnings from equity method investment		14.6	0.0	40.7
Add: Capital contributions		5.8	0.0	10.4
Less: Distributions		9.7	0.0	27.7
Less: Other		0.0	0.0	0.1
Investment in ATC, balance at end of period	$ 0.0	$ 405.5	$ 0.0	$ 405.5

Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, classified as investing activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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