Stockholders' Equity (Summary of Purchases of Common Stock) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended						18 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019
Stockholders Equity [Line Items]
Total Number of Shares Purchased				40,822			120,201
Dollar Value of Shares Purchased				$ 1,712			$ 4,161
Average Price Paid per Share				$ 41.92			$ 34.62
Open-Market [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased	5,911	36,141	23,612	16,492	11,154	2,561
Dollar Value of Shares Purchased	$ 159	$ 1,024	$ 745	$ 712	$ 439	$ 82
Average Price Paid per Share	$ 27.01	$ 28.33	$ 31.57	$ 43.13	$ 39.35	$ 32.19
ASR [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased				24,330
Dollar Value of Shares Purchased				$ 1,000
Average Price Paid per Share				$ 41.10