Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits
Beginning balance	$ 1,168	$ 954	$ 421
Increase due to current year tax positions	1,768	118	187
Increase due to prior year tax positions	16	111	369
Decrease due to prior year tax positions	(2)	(7)	(15)
Settlements	(233)	0	0
Lapse of statutes of limitations	(16)	(8)	(8)
Ending balance	2,701	1,168	954
Net amount of potential tax benefits that would impact the entity's effective tax rate	2,600	1,100
Reimbursement receivable for unrecognized tax benefits and related interest and penalties for periods after separation	85
Interest and penalties	24	35	13
Accrued interest and penalties	120	$ 112	$ 83
Reasonably possible amount that gross unrecognized tax benefits may change within the next twelve months, high end of range	$ 31