Receivables from customers, net and notes receivable, net - Allowance for Customer Receivables (Details) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017
Receivables from customers and notes receivable, net [Abstract]
Allowance For Doubtful Accounts - Deposits with and Receivables from Broker-Dealers, Clearings Organizations, and Counterparties	$ 48.0	$ 47.0
Allowance for Doubtful Accounts Receivable	$ 10.2	$ 7.6

For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.4)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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