INCOME TAXES - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory income tax rate		35.00%	35.00%	35.00%
Tax cuts and jobs act of 2017, tax rate increased deferred income tax expense		$ 3.0
Tax cuts and jobs act of 2017, decrease in accumulated deferred income tax liability		184.2
Tax cuts and jobs act of 2017, increase in regulatory liabilities		244.9
Tax cuts and jobs act of 2017, impact of deferred tax asset		60.7
Unrecognized tax benefits		$ 0.0	$ 0.0	$ 0.0
Scenario Forecast
Income Taxes [Line Items]
Federal statutory income tax rate	21.00%