Background and Basis of Presentation (Share Repurchase Table) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Total number of shares repurchased (shares)	6.2	11.1	21.8	33.2
Aggregate cost of shares repurchased	$ 367	$ 759	$ 1,317	$ 2,359	$ 2,917
Average price per share of shares repurchased (usd per share)	$ 59.18	$ 67.99	$ 60.53	$ 70.97

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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