Summary of Significant Accounting Policies - Income Tax and Common Stock (Details) $ in Millions	12 Months Ended
Sep. 30, 2017 USD ($) vote	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Class of Stock [Line Items]
Unrecognized tax benefits, interest and penalties \| $	$ 0.2	$ 0.2	$ 0.1
Common Stock Class B Voting
Class of Stock [Line Items]
Number of votes per share \| vote	1

Amount of expense for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32718-109319

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