Receivables From Customers, Net and Notes Receivable, Net Receivables from Customers, Net and Notes Receivables, Net - Bad Debt Expense and Recoveries (Details) - USD ($)	3 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Bad debts	$ 0.3	$ 0.1
(Recovery) bad debt on physical coal	$ 2.4	$ (1.0)

Amount of expense related to write-down of receivables to the amount expected to be collected. Includes, but is not limited to, accounts receivable and notes receivable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82913815&loc=SL49130534-203044

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.5)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82913847&loc=SL49130543-203045

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration