Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		6 Months Ended		125 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018
Class of Stock [Line Items]
Shares repurchased		888,383	3,412,858	40,667,402
Average purchase price per share		$ 112.56	$ 88.88	$ 59.88
Shares repurchased (in $ millions)		$ 100	$ 303	$ 2,435
Aggregate Board of Directors repurchase authorizations (in $ millions)	[1]	$ 0	$ 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.