Derivative Instruments and Hedging Activities - Narrative (Details) - gal gal in Millions	9 Months Ended
Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Volume of commodity derivative (gallons)	213.8	245.4	259.4
Maximum period of price risk cash flow hedging (in months)	27 months

Maximum period the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

Name:	us-gaap_MaximumLengthOfTimeHedgedInPriceRiskCashFlowHedge1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration