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Intangible Assets (Total Book Value of Intangible Assets ) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Notes to Consolidated Financial Statements [Abstract]    
Intangible assets having a definite useful life $ 365 $ 415
Intangible assets having an indefined useful life 357 409
Total intangible assets $ 722 $ 824