INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2024	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate, percent				24.50%
Tax Cuts And Jobs Act Of 2017, income tax benefit		$ (57.6)
Forecasted effective tax rate	(7.20%)	13.90%
Forecasted tax credits, net of deferred tax		$ 21.9
Excess tax benefits associated with vesting of share-based awards	$ 1.3	$ 2.8
Actual effective tax rate since tax effects of awards are treated as discrete item	(8.80%)	(68.20%)
Deferred tax assets, operating loss carryforwards, domestic, refundable and receivables	$ 23.0			$ 23.0
Deferred tax assets, operating loss carryforwards, domestic	24.5
ITC carryforward	$ 145.4			121.1
ITC carryforward, expiration period	20 years
Valuation allowance	$ 4.1			4.0
Production tax credit, extension term	5 years
Production tax credit, project phase out, term	3 years
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 136.8			136.8
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 577.4			$ 578.8
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate, percent			21.00%
Forecasted tax credits, net of deferred tax			$ 47.7
Investment tax credit, solar property, percentage					10.00%	22.00%	26.00%	30.00%
Minimum | State
Operating Loss Carryforwards [Line Items]
Operating loss carryforward, expiration terms	7 years
Maximum | State
Operating Loss Carryforwards [Line Items]
Operating loss carryforward, expiration terms	20 years