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Summary of Accounting Policies - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares
shares in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Employee Stock Options      
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount 46 40 47
Unvested Restricted Stock      
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]      
Antidilutive Securities Excluded From Computation Of Earnings Per Share Amount 12   5