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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2018 | Sep. 30, 2017 | Jun. 30, 2017 | Sep. 30, 2018
Income Taxes [Line Items] |  |  |  |  |  |  |
Effective tax rate (ETR) from continuing operations | 23.90% | 37.00% |  | 228.20% |  | 37.20% |
Discrete net tax benefit in connection with initial analysis of tax act |  |  |  | $ (21.4) |  |  |
Decrease in net deferred tax liabilities |  |  |  | (21.4) |  |  |
Adjustment to deferred income tax benefit |  |  |  | (21.4) |  |  |
Net operating loss carryforward | $ 411.2 |  | $ 411.2 | 411.2 |  |  |
State net operating loss carryforwards | $ 20.1 |  | 20.1 | $ 20.1 |  |  |
Estimated increase in net operating loss carryforward |  |  | $ 111.4 |  |  |  |
Domestic Tax Authority [Member] | 2027-2036 [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Net operating loss carryforwards |  |  |  |  | $ 299.8 |  |
State and Local Jurisdiction [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Operating loss carryforwards, expiration year |  |  | 2018 |  |  |  |
State and Local Jurisdiction [Member] | 2018-2037 [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Net operating loss carryforwards |  |  |  |  | $ 172.3 |  |
Subsequent Event [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Federal corporate tax rate |  |  |  |  |  |  | 21.00%
Blended corporate tax rate |  |  |  |  |  |  | 24.53%
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 |  |  |  |  | 2018 |  |
Maximum [Member] | Domestic Tax Authority [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Operating loss carryforwards, expiration year |  |  |  |  | 2036 |  |
Maximum [Member] | State and Local Jurisdiction [Member] |  |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |  |
Operating loss carryforwards, expiration year |  |  |  |  | 2037 |  |

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