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Description of Business, Basis of Presentation and Significant Accounting Policies - Aggregate Amortization Expense for Internal and External-Use Software (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Acquired Intangible Assets Amortization [Line Items]      
Actual 2017 $ 8 $ 10 $ 40
2018 7    
2019 6    
2020 4    
2021 4    
2022 2    
Internal-use software amortization expense      
Acquired Intangible Assets Amortization [Line Items]      
Actual 2017 6    
2018 7    
2019 7    
2020 7    
2021 7    
2022 6    
External-use software amortization expense      
Acquired Intangible Assets Amortization [Line Items]      
Actual 2017 69    
2018 49    
2019 34    
2020 22    
2021 0    
2022 $ 0