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Stock-Based Incentive Plans - Movement in Outstanding Shares of Restricted Class A Common Stock and Weighted-Average Grant Date Fair Value Per Share (Detail) - Restricted Class A Common Stock [Member] - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted Class A Common Stock, Beginning Balance 1,614,907 1,698,582  
Restricted Class A Common Stock, Granted 934,502 1,049,647  
Restricted Class A Common Stock, Vested (1,004,315) (948,407)  
Restricted Class A Common Stock, Forfeited (879) (17,565) (17,820)
Restricted Class A Common Stock, Ending Balance 1,627,890 1,614,907 1,698,582
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance $ 32.94 $ 34.52  
Weighted-Average Grant Date Fair Value Per Share, Granted 31.83 34.69  
Weighted-Average Grant Date Fair Value Per Share, Vested 34.26 32.08  
Weighted-Average Grant Date Fair Value Per Share, Forfeited 34.57 31.55  
Weighted-Average Grant Date Fair Value Per Share, Ending Balance $ 33.14 $ 32.94 $ 34.52