Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Liabilities:
Carrying value of debt, net of unamortized discount	$ 10,400	$ 9,900
Total Fair Value
Liabilities:
Debt estimated fair value	10,600	10,400
Recurring | Level 2 | Accrued liabilities
Liabilities:
Embedded derivatives	31	39
Recurring | Level 2 | Deferred credits and liabilities
Liabilities:
Embedded derivatives	125	147
Recurring | Total Fair Value | Accrued liabilities
Liabilities:
Embedded derivatives	31	39
Recurring | Total Fair Value | Deferred credits and liabilities
Liabilities:
Embedded derivatives	125	147
Non recurring | Proved gas and oil properties
Liabilities:
Pre-tax impairment charges	$ 30	$ 397