Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended				119 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2017
Class of Stock [Line Items]
Share repurchase plan, authorized repurchase amount	[1]	$ 3,866				$ 3,866
Shares repurchased		5,436,803	7,034,420	6,640,601	[2]	39,779,019
Average purchase price per share		$ 91.97	$ 71.08	$ 63.31		$ 58.71
Amount spent on repurchased shares (in millions)		$ 500	$ 500	$ 420		$ 2,335
Aggregate Board of Directors repurchase authorizations during the period	[1]	$ 1,500	$ 0	$ 1,000
Restricted Stock [Member]
Class of Stock [Line Items]
Shares withheld, tax withholding				9,264

[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.

[2] The year ended December 31, 2015 excludes 9,264 shares 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.