INCOME TAXES - INCOME TAX CARRYFORWARDS (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating Loss Carryforwards [Line Items]
Tax benefit associated with the loss carryforwards	$ 23,597	$ 18,113
Tax credit carryforward, expiration period	20 years
Deferred tax asset, loss carryforwards	$ 52,100
Natural Gas Distribution
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	2,300	2,500
Clean Energy Ventures
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	109,300	74,000
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating losses	125,300	78,700
Income taxes receivable	15,400
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating losses	471,700	310,600
Deferred tax asset, loss carryforwards	161,397
State and Local Jurisdiction | Fiscal years 2018 - 2022
Operating Loss Carryforwards [Line Items]
Deferred tax asset, loss carryforwards	313
State and Local Jurisdiction | Fiscal years 2023 - 2027
Operating Loss Carryforwards [Line Items]
Deferred tax asset, loss carryforwards	1,051
State and Local Jurisdiction | Fiscal years 2028 - 2032
Operating Loss Carryforwards [Line Items]
Deferred tax asset, loss carryforwards	796
State and Local Jurisdiction | Fiscal years 2033 - 2037
Operating Loss Carryforwards [Line Items]
Deferred tax asset, loss carryforwards	$ 159,237
Minimum | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating losses, life	7 years
Maximum | State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating losses, life	20 years
MONTANA
Operating Loss Carryforwards [Line Items]
Valuation allowance	$ 1,000
NEW JERSEY
Operating Loss Carryforwards [Line Items]
Valuation allowance		$ 262