Stockholders' Equity (Summary of Purchases of Common Stock) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018
Stockholders Equity [Line Items]
Total Number of Shares Purchased	40,822			54,537
Dollar Value of Shares Purchased	$ 1,712			$ 2,233
Average Price Paid per Share	$ 41.92			$ 40.94
Open-Market [Member]
Stockholders Equity [Line Items]
Total Number of Shares Purchased	16,492	11,154	2,561
Dollar Value of Shares Purchased	$ 712	$ 439	$ 82
Average Price Paid per Share	$ 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