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INCOME TAXES (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2018
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Operating Loss Carryforwards [Line Items]          
Federal statutory income tax rate, percent   24.50%      
Tax Cuts And Jobs Act Of 2017, income tax benefit $ (967) $ (58,532)      
Forecasted effective tax rate   17.20% 15.60%    
Forecasted tax credits, net of deferred tax         $ 36,400
Excess tax benefits associated with vesting of share-based awards   $ 2,800 $ 1,500    
Actual effective tax rate since tax effects of awards are treated as discrete item   (7.90%) 14.80%    
Deferred tax assets, federal 28,500 $ 28,500     28,500
Deferred tax asset, state 28,200 28,200     23,600
ITC carryforward 127,800 $ 127,800     109,300
ITC carryforward, expiration period   20 years      
Federal          
Operating Loss Carryforwards [Line Items]          
Net operating loss carryforwards 125,300 $ 125,300     125,300
State          
Operating Loss Carryforwards [Line Items]          
Net operating loss carryforwards 537,200 537,200     471,700
Scenario, Forecast          
Operating Loss Carryforwards [Line Items]          
Forecasted tax credits, net of deferred tax       $ 23,200  
Montana, Iowa, Kansas And Wyoming          
Operating Loss Carryforwards [Line Items]          
Valuation allowance 2,500 2,500      
MONTANA          
Operating Loss Carryforwards [Line Items]          
Valuation allowance         $ 1,000
Corporate, Non-Segment          
Operating Loss Carryforwards [Line Items]          
Tax Cuts And Jobs Act Of 2017, income tax benefit 749 10,723      
Natural Gas Distribution          
Operating Loss Carryforwards [Line Items]          
Tax Cuts and Jobs Act of 2017, regulatory liability 228,400 228,400      
Tax Cuts and Jobs Act of 2017, revaluation of deferred income taxes, provisional liability 164,300 164,300      
Tax Cuts and Jobs Act of 2017, income tax effects on revaluation, provisional liability 64,100 64,100      
Midstream Investments          
Operating Loss Carryforwards [Line Items]          
Tax Cuts And Jobs Act Of 2017, income tax benefit 185 (13,804)      
Energy Services          
Operating Loss Carryforwards [Line Items]          
Tax Cuts And Jobs Act Of 2017, income tax benefit (1,456) 7,651      
Clean Energy Ventures          
Operating Loss Carryforwards [Line Items]          
Tax Cuts And Jobs Act Of 2017, income tax benefit $ (445) $ (63,102)