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Capital Stock (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Millions
1 Months Ended 6 Months Ended
May 31, 2017
Jan. 31, 2017
Jun. 30, 2017
Jun. 30, 2016
Mar. 31, 2015
Class of Stock Disclosures [Abstract]          
Cash dividends declared, per share of common stock $ 0.425   $ 0.850    
Stock Repurchased During Period, Shares   1.9      
Payments for Repurchase of Common Stock   $ 230,000,000 $ 230,000,000 $ 13,000,000  
2015 $2 Billion Repurchase Agreement          
Stock Repurchase Program, Authorized Amount         $ 2,000,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount     $ 1,640,000,000