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(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 |

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