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Share-Based Compensation - Additional Information (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, authorized shares 6,000,000,000   6,000,000,000   6,000,000,000
2017 Employee Plan [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Shares of common stock reserved for future issuance 6,016,487   6,016,487    
Compensation expense $ 4,000,000 $ 4,000,000 $ 12,000,000 $ 11,000,000  
Unrecognized compensation costs related to unvested awards $ 20,000,000   $ 20,000,000    
Unrecognized compensation costs related to unvested awards, weighted-average period     1 year 4 months 24 days    
2017 Employee Plan [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, authorized shares 8,000,000   8,000,000    
2017 Director Plan [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Shares of common stock reserved for future issuance 349,793   349,793    
2017 Director Plan [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock, authorized shares 450,000   450,000    
Performance Stock Units ("PSUs") [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Market capitalization $ 1,000,000,000   $ 1,000,000,000    
Vesting rights     zero to 200%    
Performance Stock Units ("PSUs") [Member] | Maximum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share-based compensation arrangement by share-based payment award performance period     3 years    
Vesting rights     200%    
Performance Stock Units ("PSUs") [Member] | Minimum [Member]          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Share-based compensation arrangement by share-based payment award performance period     2 years    
Vesting rights     0%