Goodwill - Goodwill by Segment (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019
Goodwill [Line Items]
Goodwill	$ 52.2	$ 51.2
Commodity and Risk Management [Member]
Goodwill [Line Items]
Goodwill	30.2	30.3
Global Payments
Goodwill [Line Items]
Goodwill	7.6	7.6
Physical Commodities
Goodwill [Line Items]
Goodwill	8.7	8.7
Securities
Goodwill [Line Items]
Goodwill	4.6	4.6
Clearing and Execution Services Segment
Goodwill [Line Items]
Goodwill	$ 1.1	$ 0.0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant