(Details) $ in Thousands	6 Months Ended
	Mar. 31, 2018 USD ($) mi	Sep. 30, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in equity investees	$ 183,884	$ 172,585
Steckman Ridge
Schedule of Equity Method Investments [Line Items]
Investments in equity investees	118,875	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	$ 65,009	$ 52,323
Construction plan, project area (in miles) | mi	120