REGULATORY ASSETS AND LIABILITIES - Regulatory Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	[1]	$ 9.9	$ 6.5
Regulatory liabilities, noncurrent		441.0	189.1
Total regulatory liabilities		450.9	195.6
CET
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	[2]	4.4	2.9
Cost of plant removal and ARO's
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	[3]	4.2	3.5
Regulatory liabilities, noncurrent	[3]	194.0	189.1
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current		1.3	$ 0.1
Regulatory liabilities, noncurrent		2.1
Income taxes refundable through future rates
Regulatory Liabilities [Line Items]
Regulatory liabilities, noncurrent	[4]	$ 244.9

[1] Current regulatory liabilities are presented in other current liabilities in the Balance Sheets.

[2] Represents the difference between actual and allowed revenues. Any deficiency in amounts collected are recovered through periodic rate adjustments.

[3] Cost of plant removal and AROs represent amounts recovered from customers for costs of future activities to remove assets that are expected to be incurred at the time of retirement.

[4] Amounts recorded to pass the effect of reduced income tax rates from the 2017 Tax Reform Act to customers in future periods, which will reverse at the weighted average tax rate that was used to build the reserves over the remaining book life of the property