Commitments and Contingencies - Purchase and sales commitments (Details) bbl in Thousands, $ in Thousands	Sep. 30, 2018 USD ($) bbl
Fixed Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) \| bbl	1,469
Sale commitments, Value \| $	$ 102,801
Floating Price Sales [Member]
Summary Of Purchase And Sale Commitments
Sale commitments, Volume (barrels) \| bbl	12,225
Sale commitments, Value \| $	$ 728,646
Fixed Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) \| bbl	1,469
Purchase commitments, Value \| $	$ 102,956
Floating Price Purchases [Member]
Summary Of Purchase And Sale Commitments
Purchase commitments, Volume (barrels) \| bbl	8,506
Purchase commitments, Value \| $	$ 632,046

The floor amount as of the balance sheet date that the entity must expend to satisfy the terms of disclosed arrangements (excluding long-term commitments) in which the entity must commit resources to supply goods or services to one or more customers.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.(a)(19))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.17)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.25)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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