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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

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