Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Unrecognized tax benefits
Unrecognized tax benefits, tax benefits that if recognized would impact the effective tax rate	$ 3.0
Interest related to unrecognized benefits
Interest related to unrecognized benefits	$ 0.1	$ 0.1	$ 0.1

The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant