Revenue Recognition (Disaggregation of Revenue) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Total revenue derived from contracts with purchasers	$ 2,336
Oil sales
Disaggregation of Revenue [Line Items]
Total revenue derived from contracts with purchasers	1,013
NGL sales
Disaggregation of Revenue [Line Items]
Total revenue derived from contracts with purchasers	165
Gas sales
Disaggregation of Revenue [Line Items]
Total revenue derived from contracts with purchasers	88
Total oil and gas sales
Disaggregation of Revenue [Line Items]
Total revenue derived from contracts with purchasers	1,266
Sales of purchased oil and gas
Disaggregation of Revenue [Line Items]
Total revenue derived from contracts with purchasers	$ 1,070