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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2016
Income Taxes [Line Items] |  |  |
Operating loss carryforwards, valuation allowance | $ 1.9 | $ 2.6 |
Unrecognized tax benefits if recognized would result in adjustments to primarily deferred taxes | $ 4.7 | $ 7.5 | $ 7.5
Federal corporate tax rate | 21.00% |  |
Blended corporate tax rate | 24.50% |  |
Discrete net tax benefit in connection with initial analysis of tax act | $ 22.0 |  |
Decrease in net deferred tax liabilities excluding valuation allowance | 22.0 |  |
Estimated increase to valuation allowance | 0.5 |  |
Adjustment to deferred income tax benefit | 21.5 |  |
AMT credits | 2.5 |  |
Accounting Standards Update 2016-09 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Nondeductible stock compensation expenses | 0.4 |  |
Domestic Tax Authority [Member] | 2027-2037 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Net operating loss carryforwards | 415.1 |  |
State and Local Jurisdiction [Member] | 2019-2038 [Member] |  |  |
Income Taxes [Line Items] |  |  |
Net operating loss carryforwards | $ 199.6 |  |
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 | 2019 |  |
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 | 2038 |  |

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