Acquisitions - Other insignficant acquisitions (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Mar. 31, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Estimated Income tax rate	21.00%	35.00%
Oil and natural gas properties
Goodwill (Note 12)	$ 2,280.2	$ 2,277.1
Minimum [Member] \| Customer Relationships [Member]
Oil and natural gas properties
Amortization period, intangible asset	2 years
Maximum \| Customer Relationships [Member]
Oil and natural gas properties
Amortization period, intangible asset	20 years

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=64834752&loc=SL49117168-202975

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