Commitments - Additional Information (Detail) $ in Millions	12 Months Ended
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

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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

Name:	us-gaap_OperatingLeasesRentExpenseNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration