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Segment and Georgraphic Data - Additional Information (Detail) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2017 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($)
Segment Reporting Information [Line Items] |  |  |
Consolidated operating income | $ 283.0 | $ 334.9 | $ (48.8)
Number of reporting segments | 2 |  |
Intersegment Eliminations [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Consolidated operating income | $ 0.6 | $ 0.0 | $ 0.0

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The net result for the period of deducting operating expenses from operating revenues. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=us-gaap_IntersegmentEliminationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |