Revenue Recognition (Schedule of Revenue from Contracts with Customers that are Disaggregated Based on Type of Good or Service) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	$ 1,566		$ 2,926
Marketing revenues	1,180	$ 833	2,059	$ 1,692
Total revenues from contracts with customers	2,746		4,985
Oil [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	1,121		2,028
Gas [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	207		462
NGL [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	238		436
Operating Segments [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	1,253		2,383
Total revenues from contracts with customers	2,409		4,401
Operating Segments [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	313		543
Total revenues from contracts with customers	337		584
Operating Segments [Member] | Oil [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	808		1,485
Operating Segments [Member] | Oil [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	313		543
Operating Segments [Member] | Gas [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	207		462
Operating Segments [Member] | NGL [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Oil, gas and NGL sales	238		436
Marketing Revenues [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	1,180		2,059
Marketing Revenues [Member] | Oil [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	790		1,338
Marketing Revenues [Member] | Gas [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	160		315
Marketing Revenues [Member] | NGL [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	230		406
Marketing Revenues [Member] | Operating Segments [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	1,156		2,018
Marketing Revenues [Member] | Operating Segments [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	24		41
Marketing Revenues [Member] | Operating Segments [Member] | Oil [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	766		1,297
Marketing Revenues [Member] | Operating Segments [Member] | Oil [Member] | Canada [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	24		41
Marketing Revenues [Member] | Operating Segments [Member] | Gas [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	160		315
Marketing Revenues [Member] | Operating Segments [Member] | NGL [Member] | U.S. [Member]
Disaggregation Of Revenue [Line Items]
Marketing revenues	$ 230		$ 406