Stockholders' Equity (Schedule of Treasury Stock) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		146 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020
Class of Stock [Line Items]
Shares repurchased	1,709,431	1,972,291	58,588,409
Average purchase price per share	$ 87.87	$ 101.41	$ 73.44
Shares repurchased (in $ millions)	$ 150	$ 200	$ 4,303
Aggregate Board of Directors repurchase authorizations (in $ millions)	$ 0	$ 0	$ 5,366