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Commitments - Additional Information (Detail) $ in Millions | 12 Months Ended
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Sep. 30, 2018 USD ($) executive | Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($)
Commitments and Contingencies Disclosure [Abstract] |  |  |
Rent expense under operating leases | $ | $ 19.8 | $ 18.6 | $ 17.6
Number of executives in a management agreement providing for certain payments and other benefits in the event of a qualified change | executive | 22 |  |

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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
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