Segment Information (Schedule of Revenue from Contracts with Customers Disaggregated Based on Type of Good) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									$ 4,449		$ 3,571	$ 2,772
Oil, gas and NGL derivatives									608		157	(201)
Upstream revenues									6,285		5,307	3,981
Total revenues	$ 3,708	$ 2,579	$ 2,249	$ 2,198	$ 2,380	$ 1,933	$ 2,165	$ 2,400	10,734	[1]	8,878	6,753
Operating Segments [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL derivatives									457
Upstream revenues									5,320
Total revenues									9,674	[1]	7,326	5,722
Operating Segments [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL derivatives									151
Upstream revenues									965
Total revenues									1,060	[1]	$ 1,552	$ 1,031
Oil, Gas and NGL Sales [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									5,677
Oil, Gas and NGL Sales [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									3,771
Oil, Gas and NGL Sales [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									950
Oil, Gas and NGL Sales [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									956
Oil, Gas and NGL Sales [Member] | Operating Segments [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									4,863
Oil, Gas and NGL Sales [Member] | Operating Segments [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									814
Oil, Gas and NGL Sales [Member] | Operating Segments [Member] | Oil [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									2,957
Oil, Gas and NGL Sales [Member] | Operating Segments [Member] | Oil [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									814
Oil, Gas and NGL Sales [Member] | Operating Segments [Member] | Gas [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									950
Oil, Gas and NGL Sales [Member] | Operating Segments [Member] | NGL [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									956
Marketing Revenues [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									4,449
Marketing Revenues [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									2,840
Marketing Revenues [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									738
Marketing Revenues [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									871
Marketing Revenues [Member] | Operating Segments [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									4,354
Marketing Revenues [Member] | Operating Segments [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									95
Marketing Revenues [Member] | Operating Segments [Member] | Oil [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									2,745
Marketing Revenues [Member] | Operating Segments [Member] | Oil [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									95
Marketing Revenues [Member] | Operating Segments [Member] | Gas [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									738
Marketing Revenues [Member] | Operating Segments [Member] | NGL [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Total revenues from contracts with customers									$ 871

[1] Revenues from oil, gas and NGL sales and marketing revenues represent revenue from contracts with customers.