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Fair Value Measurements (Assets and Liabilities Measured At Fair Value On A Recurring Basis Using Unobservable Inputs (Level 3)) (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Fair Value Disclosures [Abstract]    
Estimated fair value due in one year or less $ 50 $ 36
Estimated fair value due after one year through three years $ 47 $ 32