Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		9 Months Ended		116 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Jul. 17, 2017
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount					$ 1,500
Shares repurchased		5,436,803	4,360,617	39,779,019
Average purchase price per share		$ 91.97	$ 68.80	$ 58.71
Shares repurchased (in $ millions)		$ 500	$ 300	$ 2,335
Aggregate Board of Directors repurchase authorizations (in $ millions)	[1]	$ 1,500	$ 0	$ 3,866

[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.