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Summary of Significant Accounting Policies - Allowance for Doubtful Accounts Roll-forward (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accounting Policies [Abstract]      
Allowance for doubtful accounts at beginning of year $ 103 $ 98 $ 78
Additional provisions for the year 14 40 4
Uncollectible accounts written off net of recoveries (24) (35) 16
Allowance for doubtful accounts at end of year $ 93 $ 103 $ 98