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VARIABLE INTEREST ENTITIES (Details) $ in Millions	3 Months Ended
VARIABLE INTEREST ENTITIES (Details) $ in Millions	Mar. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Variable interest entities
Equity investment	$ 1,670.6	$ 1,665.3	$ 1,598.9	$ 1,553.4
Firm capacity from purchased power agreement (in megawatts) \| MW	236
Minimum energy requirements over remaining term of purchased power agreement (in megawatts) \| MW	0
ATC
Variable interest entities
Ownership interest (as a percent)	60.00%
Equity investment	$ 1,630.6	1,625.3
Construction costs to be reimbursed to UMERC	32.4	29.4
Accounts payable due to ATC	$ 29.0	$ 28.2
ATC Holdco
Variable interest entities
Ownership interest (as a percent)	75.00%
Equity investment	$ 40.0
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)	3 years
Residual guarantee associated with purchased power agreement	$ 0.0
Required payments over remaining term of purchased power agreement	$ 28.6