INVESTMENTS IN EQUITY INVESTEES (Details) $ in Thousands		6 Months Ended
		Mar. 31, 2019 USD ($) mi	Sep. 30, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in equity investees		$ 195,203	$ 190,866
Steckman Ridge
Schedule of Equity Method Investments [Line Items]
Investments in equity investees	[1]	115,775	117,001
Total outstanding principal balance of loans		70,400	70,400
PennEast
Schedule of Equity Method Investments [Line Items]
Investments in equity investees		$ 79,428	$ 73,865
Construction plan, project area (in miles) | mi		120

[1] Includes loans with a total outstanding principal balance of $70.4 million for both March 31, 2019 and September 30, 2018. The loans accrue interest at a variable rate that resets quarterly and are due October 1, 2023.