Stockholders' Equity (Summary of Purchases of Common Stock) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended					12 Months Ended	24 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019
5.0 Billion Dollar Share Repurchase Program [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased		4,436	22,137	5,911	36,141	78,149	146,774
Dollar Value of Shares Purchased		$ 94	$ 550	$ 159	$ 1,024	$ 2,978	$ 4,805
Average Price Paid per Share		$ 21.32	$ 24.80	$ 27.01	$ 28.33	$ 38.11	$ 32.74
1.0 Billion Dollar Share Repurchase Program [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased	2,243
Dollar Value of Shares Purchased	$ 38
Average Price Paid per Share	$ 16.85