VARIABLE INTEREST ENTITIES (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) MW	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Variable interest entities
Equity investment	$ 1,598.9	$ 1,513.3	$ 1,553.4	$ 1,443.9
ATC
Variable interest entities
Ownership interest (as a percent)	60.00%
Equity investment	$ 1,561.1		1,515.8
ATC distributions receivable			39.9
Accounts payable due to ATC	$ 24.0		31.2
ATC Holdco
Variable interest entities
Ownership interest (as a percent)	75.00%
Equity investment	$ 37.8		$ 37.6
Purchased power agreement
Variable interest entities
Firm capacity from purchased power agreement (in megawatts) | MW	236
Minimum energy requirements over remaining term of purchased power agreement (in megawatts) | MW	0
Remaining term of purchased power agreement (in years)	4 years
Residual guarantee associated with purchased power agreement	$ 0.0
Required payments over remaining term of purchased power agreement	67.7
Total capacity and lease payments	$ 4.7	$ 4.5