Physical Commodities Inventory - LCM Adjustments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Physical Commodities Inventory [Abstract]
Inventory Adjustments	$ 0.1	$ 0.7	$ 0.6

Amount of inventory reserves for last-in first-out (LIFO) and other inventory valuation methods.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 330
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66092785&loc=d3e4492-108314

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 210.5-02.6(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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