Stockholders' Equity (Summary of Purchases of Common Stock) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended			24 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
5.0 Billion Dollar Share Repurchase Program [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased		68,625	78,149		146,774
Dollar Value of Shares Purchased		$ 1,827	$ 2,978		$ 4,805
Average Price Paid per Share		$ 26.62	$ 38.11		$ 32.74
1.0 Billion Dollar Share Repurchase Program [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased	2,243			2,243
Dollar Value of Shares Purchased	$ 38			$ 38
Average Price Paid per Share	$ 16.85			$ 16.85