Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended				131 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
Shares reacquired under clawback agreement		1,700
Shares repurchased		7,933,692	[1]	5,436,803	7,034,420	47,712,711
Average purchase price per share		$ 103.01		$ 91.97	$ 71.08	$ 66.08
Amount spent on repurchased shares (in millions)		$ 817		$ 500	$ 500	$ 3,153
Aggregate Board of Directors repurchase authorizations during the period	[2]	0		$ 1,500	$ 0
Share repurchase plan, authorized repurchase amount	[2]	$ 3,866				$ 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.