Stockholders' Equity - Repurchases of Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended		12 Months Ended
	Apr. 30, 2016	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2015
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value			$ 954,000,000	$ 11,001,000,000	$ 10,002,000,000
Stock Repurchase, Average Cost Per Share			$ 71.79	$ 89.15	$ 104.91
2016 Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		$ 12,000,000,000.0
Stock Repurchase Program, Remaining Authorized Repurchase Amount			$ 8,000,000,000
Stock Repurchased and Retired During Period, Shares				36
Stock Repurchased and Retired During Period, Value				$ 3,000,000,000
2016 Stock Repurchase Program [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares			13
Stock Repurchased and Retired During Period, Value			$ 954,000,000
2015 Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount		5,000,000,000				$ 15,000,000,000
Accelerated Share Repurchases, Settlement (Payment) or Receipt		$ 5,000,000,000
Accelerated Share Repurchases, Initial Price Paid Per Share		$ 86.68
Stock Repurchased and Retired During Period, Shares	8	46		87	65
Stock Repurchase Program, Percentage of Shares Repurchased		80.00%
Stock Repurchased and Retired During Period, Value				$ 8,000,000,000	$ 7,000,000,000
Accelerated Share Repurchases, Final Price Paid Per Share	$ 92.09
2014 Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount					$ 5,000,000,000
Stock Repurchased and Retired During Period, Shares					30
Stock Repurchased and Retired During Period, Value					$ 3,000,000,000
Publicly Announced Program [Member]
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Shares			13	123	95