DERIVATIVE INSTRUMENTS - INCOME STATEMENT RELATED DISCLOSURES (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Foreign currency contracts | Gas purchases | Derivatives in cash flow hedging relationships:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain or (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 0	$ (27)
Amount of Gain or (Loss) Reclassified from OCI into Income (Effective Portion)	0	27
Amount of Gain or (Loss) Recognized on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0	0
Energy Services | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	8,055	32,942	$ 107,212
Energy Services | Physical commodity contracts | Operating revenues | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	8,912	33,034	32,568
Energy Services | Physical commodity contracts | Gas purchases | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(27,461)	(45,637)	(34,438)
Energy Services | Financial commodity contracts | Gas purchases | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	26,563	45,579	109,082
Energy Services | Foreign currency contracts | Gas purchases | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	41	(34)	0
Natural Gas Distribution | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	7,898	(42,585)	(37,656)
Natural Gas Distribution | Physical commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives			0
Natural Gas Distribution | Physical commodity contracts | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(12,303)	(15,756)
Natural Gas Distribution | Financial commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives			(33,428)
Natural Gas Distribution | Financial commodity contracts | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	5,595	(7,984)
Natural Gas Distribution | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives			$ (4,228)
Natural Gas Distribution | Interest rate contracts | Derivatives not designated as hedging instruments:
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	$ 14,606	$ (18,845)