Goodwill - Goodwill by Segment (Details) - USD ($) $ in Millions	Jun. 30, 2019	Sep. 30, 2018
Goodwill [Line Items]
Goodwill	$ 53.7	$ 48.4
Commercial Hedging [Member]
Goodwill [Line Items]
Goodwill	30.3	30.3
Global Payments [Member]
Goodwill [Line Items]
Goodwill	6.9	8.9
Physical Commodities [Member]
Goodwill [Line Items]
Goodwill	9.5	2.4
Securities [Member]
Goodwill [Line Items]
Goodwill	$ 7.0	$ 6.8

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