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Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Income Taxes [Line Items] |  |  |  |
Operating loss carryforwards, valuation allowance | $ 1,900 | $ 1,900 | $ 1,900 |
Unrecognized tax benefits if unrecognized, the balance of uncertain tax benefit would affect the effective tax rate | 4,700 | $ 4,700 | $ 4,700 | $ 7,500
Federal corporate tax rate |  | 21.00% | 21.00% |
Discrete net tax benefit in connection with initial analysis of tax act |  |  | $ 22,400 |
Decrease in net deferred tax liabilities excluding valuation allowance |  |  | 22,000 |
Estimated increase to valuation allowance |  |  | 500 |
Adjustment to deferred income tax benefit | 900 |  | 21,500 |
AMT credits | $ 2,500 |  | 2,500 |
Provisional tax expense (benefit) |  | $ 15,706 | $ (17,426) | $ 20,874
AMT Credit Carryover [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Provisional tax expense (benefit) |  | (100) |  |
Domestic Tax Authority [Member] | 2027-2037 [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Net operating loss carryforwards |  | 478,300 |  |
State and Local Jurisdiction [Member] | 2019-2038 [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Net operating loss carryforwards |  | $ 228,300 |  |
Minimum [Member] | Domestic Tax Authority [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2027 |  |
Minimum [Member] | State and Local Jurisdiction [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2020 |  |
Maximum [Member] | Domestic Tax Authority [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2037 |  |
Maximum [Member] | State and Local Jurisdiction [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Operating loss carryforwards, expiration year |  | 2039 |  |

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