Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		6 Months Ended		113 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017
Class of Stock [Line Items]
Shares repurchased		3,412,858	2,821,140	37,755,074
Average purchase price per share		$ 88.88	$ 70.89	$ 56.65
Shares repurchased (in $ millions)		$ 303	$ 200	$ 2,138
Aggregate Board of Directors repurchase authorizations (in $ millions)	[1]	$ 0	$ 0	$ 2,366

[1] These authorizations give management discretion in determining the timing and conditions under which shares may be repurchased. This repurchase program began in February 2008 and does not have an expiration date.