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Share Repurchase Programs (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Millions
 2 Months Ended 12 Months Ended
Feb. 22, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jul. 20, 2017
Equity, Class of Treasury Stock [Line Items]          
Authorized amount for stock repurchase         $ 4,000
Remaining amount available for stock repurchase   $ 3,895      
Purchases of treasury stock (in shares)   1,000,000 0 5,000,000  
Average cost of shares repurchased (in dollars per share)   $ 93.24 $ 0.00 $ 66.10  
Subsequent event [Member]          
Equity, Class of Treasury Stock [Line Items]          
Purchases of treasury stock (in shares) 4,000,000        
Payments for repurchase of shares $ 401        
Average cost of shares repurchased (in dollars per share) $ 97.70