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Revenues by Geography, Concentration of Credit Risk and Remaining Performance Obligations - Additional Information 1 (Detail)
$ in Billions
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Customer
Sep. 30, 2017
Customer
Sep. 30, 2018
USD ($)
Customer
Sep. 30, 2017
Customer
Revenue From Contract With Customer [Abstract]        
Number of clients served | Customer 0 0 0 0
Revenue expected to be recognized in future from remaining performance obligations | $ $ 18.3   $ 18.3