Other Operating Expenses - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2018	Jun. 30, 2017
Natural gas [Member]
Property, Plant and Equipment [Line Items]
Loss on contract termination	$ 23	$ 126
Oil [Member]
Property, Plant and Equipment [Line Items]
Loss on contract termination		$ 290

The loss recognized on termination of a contract.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 912
-SubTopic 275
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84167019&loc=d3e54681-109401

Name:	us-gaap_LossOnContractTermination
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration