Investment in Cenovus Energy (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	5 Months Ended	12 Months Ended
	May 17, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	Jan. 01, 2018	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Investment in Cenovus Energy		$ 1,462	$ 1,899
Accumulated Other Comprehensive Income (Loss), Net of Tax		(6,063)	(5,518)		$ (6,193)		$ (6,247)
Unrealized Gain (Loss) on Securities, before tax		$ 0	(58)	[1]	$ 0
Foster Creek Christina Lake (FCCL) and western Canada gas properties [Member]
Schedule of Available-for-sale Securities [Line Items]
Disposal Group, Including Discontinued Operation, Description and Timing of Disposal		On May 17, 2017, we completed the sale of our 50 percent nonoperated interest in the FCCL Partnership, as well as the majority of our western Canada gas assets, to Cenovus Energy.
Foster Creek Christina Lake (FCCL) [Member]
Schedule of Available-for-sale Securities [Line Items]
Ownership percentage in equity investment	50.00%
Adjustments for New Accounting Pronouncement [Member]
Schedule of Available-for-sale Securities [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax			(5,518)			$ (5,460)
Accounting Standards Update 2016-01 [Member]
Schedule of Available-for-sale Securities [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax						$ 58
Common Stock [Member] | Cenovus Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Shares Exchanged for Assets Held for Sale	208
Percent of issued and outstanding common shares of Cenovus	16.90%
Investment In Equity Security Fair Value	$ 1,960	$ 1,460	$ 1,900
Cenovus Energy Investment at Cost	$ 1,960
Cenovus Energy common share price	$ 9.41	$ 7.03
Unrealized Gain (Loss) on Securities, before tax		$ (437)

[1] **See Note 2—Changes in Accounting Principles and Note 20—Accumulated Other Comprehensive Loss, for additional information relating to the adoption of ASU No. 2016-01.