Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		9 Months Ended		104 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Class of Stock [Line Items]
Shares repurchased		4,360,617	6,640,601	31,668,413
Average purchase price per share		$ 68.80	$ 63.31	$ 51.64
Cash paid for repurchased shares (in millions)		$ 300	$ 420	$ 1,635
Aggregate Board of Directors repurchase authorizations (in millions)	[1]	$ 0	$ 1,000	$ 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.