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Revenue - Customer Contracts (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Revenue from Contract with Customer [Abstract]    
Trade receivables, net $ 1,562 $ 1,624
Contract assets (current) 107 108
Contract assets (non-current), included in other noncurrent assets 111 118
Deferred revenue (current) 842 776
Deferred revenue (non-current) $ 105 $ 106