Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended						95 Months Ended
		Sep. 09, 2015	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2015
Class of Stock [Line Items]
Share repurchase plan, authorized repurchase amount	[1]		$ 2,366						$ 2,366
Shares repurchased			6,640,601	[2]	4,338,488	[2]	3,186,180	[2]	27,307,796	[1]
Average purchase price per share			$ 63.31		$ 57.61		$ 51.38		$ 48.90
Amount spent on repurchased shares (in millions)			$ 420		$ 250		$ 164		$ 1,335
Aggregate Board of Directors repurchase authorizations during the period		$ 1,000	$ 1,000	[1]	$ 473	[1]	$ 0	[1]
Restricted Stock [Member]
Class of Stock [Line Items]
Shares withheld, tax withholding			9,264				6,021

[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. In September 2015, the Board of Directors approved a new $1.0 billion share repurchase authorization.

[2] The years ended December 31, 2015 and 2013 exclude 9,264 and 6,021 shares, respectively, withheld from an executive officer to cover statutory minimum withholding requirements for personal income taxes related to the vesting of restricted stock. Restricted stock awards are considered outstanding at the time of issuance. Accordingly, the shares withheld are treated as treasury shares.