INCOME TAXES (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	May 22, 2018
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate, percent				24.50%
Public utilities, revaluation of deferred income taxes, liability								$ 20,100
Public utilities, overcollection of taxes and interest, liability								9,000
Public utilities, remeasurement of deferred income tax, liability								14,300
Public utilities, income tax effects on revaluation								5,800
Tax Cuts And Jobs Act Of 2017, incomplete accounting, change in tax rate, deferred tax liability		$ 988		$ 988
Income tax (benefit) provision		(28,534)	$ (5,816)	(47,801)	$ 20,134
Regulatory liabilities-noncurrent		211,431		211,431			$ 14,507
Tax Cuts And Jobs Act Of 2017, income tax benefit		$ 844		(57,688)
Forecasted effective tax rate		14.30%			13.10%
Forecasted tax credits, net of deferred tax							36,400
Excess tax benefits associated with vesting of share-based awards				$ 2,900	$ 4,600
Actual effective tax rate since tax effects of awards are treated as discrete item				(23.70%)	10.70%
Deferred tax assets, federal		$ 28,500		$ 28,500			28,500
Deferred tax asset, state		33,800		33,800			23,600
ITC carryforward		130,300		$ 130,300			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		494,100		494,100			471,700
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Forecasted tax credits, net of deferred tax						$ 22,000
MONTANA
Operating Loss Carryforwards [Line Items]
Valuation allowance		2,500		2,500			1,000
Corporate, Non-Segment
Operating Loss Carryforwards [Line Items]
Income tax (benefit) provision		1,122	$ 853	11,539	$ 1,674
Tax Cuts And Jobs Act Of 2017, income tax benefit		59		10,782
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		(142)		(13,946)
Energy Services
Operating Loss Carryforwards [Line Items]
Tax Cuts And Jobs Act Of 2017, income tax benefit		1,598		9,249
Clean Energy Ventures
Operating Loss Carryforwards [Line Items]
Tax Cuts And Jobs Act Of 2017, income tax benefit		(671)		(63,773)
March 2018 Required BPU Filing
Operating Loss Carryforwards [Line Items]
Requested customer refunds, amount	$ 31,000
Deferred income taxes
Operating Loss Carryforwards [Line Items]
Income tax (benefit) provision				(16,100)
Public utilities, amortization of excess deferred income taxes, liability								890
Public utilities, revaluation of deferred income taxes not included in base rates, liability								$ 880
Regulatory liabilities-noncurrent		$ 206,832		$ 206,832			$ 0