Revenue Recognition - Impact of Adoption on Balance Sheet and Statement of Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other current liabilities	$ 1,357		$ 1,455
Other long-term liabilities	345		354
Accumulated deficit	(16,240)		$ (16,525)
Marketing revenues	1,246	$ 1,284
Marketing operating expenses	1,268	$ 1,328
Calculated under revenue guidance in effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other current liabilities	1,355
Other long-term liabilities	339
Accumulated deficit	(16,232)
Marketing revenues	1,361
Marketing operating expenses	1,383
Difference between revenue guidance in effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Other current liabilities	2
Other long-term liabilities	6
Accumulated deficit	(8)
Marketing revenues	(115)
Marketing operating expenses	$ (115)