DERIVATIVE INSTRUMENTS - INCOME STATEMENT RELATED DISCLOSURES (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Energy Services
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	$ (47,532)	$ (37,753)
Energy Services | Physical commodity contracts | Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	1,210	1,743
Energy Services | Physical commodity contracts | Gas purchases
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(22,697)	(8,799)
Energy Services | Financial commodity contracts | Gas purchases
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(25,997)	(30,611)
Energy Services | Foreign currency contracts | Gas purchases
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(48)	(86)
Natural Gas Distribution
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(15,851)	32,599
Natural Gas Distribution | Physical commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(2,976)	1,050
Natural Gas Distribution | Financial commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	(8,808)	11,178
Natural Gas Distribution | Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in income on derivatives	$ (4,067)	$ 20,371