DERIVATIVE INSTRUMENTS - INCOME STATEMENT RELATED DISCLOSURES (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	$ (21,318)	$ (14,458)	$ 38,616	$ 73,224
Energy Services \| Physical commodity contracts \| Operating revenues
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	997	(18,061)	42,668	3,871
Energy Services \| Physical commodity contracts \| Natural gas purchases
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	1,100	1,014	(1,304)	(2,288)
Energy Services \| Financial commodity contracts \| Natural gas purchases
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	(23,426)	2,408	(2,986)	71,820
Energy Services \| Foreign currency contracts \| Natural gas purchases
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	11	181	238	(179)
Natural Gas Distribution
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	15,882	2,973	13,688	(15,192)
Natural Gas Distribution \| Physical commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	399	308	1,832	1,357
Natural Gas Distribution \| Financial commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	15,483	1,029	11,856	(11,493)
Natural Gas Distribution \| Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Amount of (losses) gains recognized in income on derivatives	$ 0	$ 1,636	$ 0	$ (5,056)