INVESTMENTS IN EQUITY METHOD INVESTEES (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2018 USD ($) mi	Sep. 30, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in equity investees	$ 187,808	$ 172,585
Steckman Ridge
Schedule of Equity Method Investments [Line Items]
Investments in equity investees	117,639	120,262
Steckman Ridge | Equity Method Investee
Schedule of Equity Method Investments [Line Items]
Total outstanding principal balance of loans	70,400	70,400
PennEast
Schedule of Equity Method Investments [Line Items]
Investments in equity investees	$ 70,169	$ 52,323
Construction plan, project area (in miles) | mi	120