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Debt (Narrative) (Detail) $ in Millions | 9 Months Ended | 18 Months Ended |
--- | --- | --- | ---
Dec. 31, 2016 | Sep. 30, 2016 USD ($) | Jun. 30, 2018 | Jun. 30, 2016 USD ($)
Line Of Credit Facility [Line Items] |  |  |  |
Credit facility borrowing base |  | $ 20.0 |  | $ 20.0
Debt service coverage ratio minimum at each quarter |  | 1.2 |  |
Notes Payable, Fair Value Disclosure |  | $ 15.0 |  |
Commitment fee percentage |  | 2.30% |  |
Maximum [Member] |  |  |  |
Line Of Credit Facility [Line Items] |  |  |  |
Ratio of net debt to EBITDAX | 5.0 |  |  |
Maximum [Member] | Scenario, Forecast [Member] |  |  |  |
Line Of Credit Facility [Line Items] |  |  |  |
Ratio of net debt to EBITDAX |  |  | 3.0 |
Senior Tranche [Member] |  |  |  |
Line Of Credit Facility [Line Items] |  |  |  |
Debt instrument, commitment fee |  | 1.50% |  |
Line Of Credit Facility Maximum Borrowing Capacity |  | $ 50.0 |  |
Subordinated Tranche [Member] |  |  |  |
Line Of Credit Facility [Line Items] |  |  |  |
Debt instrument, commitment fee |  | 2.30% |  |
Line Of Credit Facility Maximum Borrowing Capacity |  | $ 15.0 |  |
Additional Term Loan Available Until Dec. 31, 2016 [Member] |  |  |  |
Line Of Credit Facility [Line Items] |  |  |  |
Notes Payable to Bank |  | $ 5.0 |  |
Term Loan [Member] |  |  |  |
Line Of Credit Facility [Line Items] |  |  |  |
Debt instrument interest rate spread |  | 5.75% |  |
Notes Payable to Bank |  | $ 15.0 |  |

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