Stockholders' Equity - Repurchases of Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 28, 2016	Jan. 31, 2015
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 2,900	$ 954	$ 11,001
Stock Repurchase, Average Cost Per Share	$ 72.95	$ 71.79	$ 89.15
2016 Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 12,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 5,100
Stock Repurchased and Retired During Period, Shares		36
Stock Repurchased and Retired During Period, Value		$ 3,000
2015 Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount					$ 15,000
Stock Repurchased and Retired During Period, Shares		87
Stock Repurchased and Retired During Period, Value		$ 8,000
Publicly Announced Program [Member]
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	40	13	123