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Stock Plans - Share Repurchase Program (Details) - USD ($)
$ / shares in Units, shares in Millions
 6 Months Ended 12 Months Ended 60 Months Ended
Jan. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2013
Class of Stock [Line Items]          
Common shares repurchased   $ 1,150,000,000 $ 2,202,000,000    
Common Stock          
Class of Stock [Line Items]          
Number of shares repurchased (in shares)   27.6      
Average cost of shares repurchased (in dollars per share)   $ 41.65      
Common shares repurchased   $ 1,200,000,000      
Stock repurchase remaining amount   $ 5,500,000,000      
Common Stock | Share Repurchase Program amended July 29, 2015          
Class of Stock [Line Items]          
Stock repurchase value $ 19,700,000,000        
Increase in share repurchase value $ 6,000,000,000        
Stock repurchase expiration date   Dec. 31, 2020      
Common Stock | Prior to January 1, 2018          
Class of Stock [Line Items]          
Stock repurchase value         $ 13,700,000,000
Common shares repurchased       $ 13,000,000,000