VARIABLE INTEREST ENTITIES (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($) MW	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Variable interest entities
Equity investment in ATC	$ 1,513.3		$ 1,443.9
ATC
Variable interest entities
Ownership interest in ATC (as a percent)	60.00%
Equity investment in ATC	$ 1,513.3		1,443.9
Accounts payable due to ATC	$ 29.1		$ 28.7
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)	5 years
Residual guarantee associated with purchased power agreement	$ 0.0
Required payments over remaining term of purchased power agreement	81.9
Total capacity and lease payments	$ 4.5	$ 13.5