Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		9 Months Ended			128 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018
Class of Stock [Line Items]
Shares repurchased		2,179,058	[1]	5,436,803	41,958,077
Average purchase price per share		$ 114.73		$ 91.97	$ 61.62
Shares repurchased (in $ millions)		$ 250		$ 500	$ 2,585
Aggregate Board of Directors repurchase authorizations (in $ millions)	[2]	$ 0		$ 1,500	$ 3,866

[1] Excludes 1,700 common shares reacquired pursuant to an employee clawback agreement.

[2] 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.