INCOME TAXES (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017
Operating Loss Carryforwards [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	24.50%
Tax Cuts And Jobs Act Of 2017, income tax benefit	$ (57.6)
Forecasted effective tax rate	13.90%	8.70%
Forecasted tax credits, net of deferred tax				$ 36.4
Excess tax benefits associated with vesting of share-based awards	$ 2.8	$ 1.2
Actual effective tax rate since tax effects of awards are treated as discrete item	(68.20%)	5.50%
Deferred tax assets, federal	$ 28.5			28.5
Deferred tax asset, state	27.7			23.6
ITC carryforward	$ 122.9			109.3
ITC carryforward, expiration period	20 years
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 125.3			125.3
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	526.4			471.7
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Forecasted tax credits, net of deferred tax			$ 21.9
MONTANA
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ 1.3			$ 1.0