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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, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Restricted Class A Common Stock, Beginning Balance 1,484,175 1,309,995
Restricted Class A Common Stock, Granted 1,001,267 925,319
Restricted Class A Common Stock, Vested (852,715) (748,016)
Restricted Class A Common Stock, Forfeited (17,820) (3,123)
Restricted Class A Common Stock, Ending Balance 1,614,907 1,484,175
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance $ 30.61 $ 28.68
Weighted-Average Grant Date Fair Value Per Share, Granted 33.93 31.78
Weighted-Average Grant Date Fair Value Per Share, Vested 30.10 28.69
Weighted-Average Grant Date Fair Value Per Share, Forfeited 30.44 27.37
Weighted-Average Grant Date Fair Value Per Share, Ending Balance $ 32.94 $ 30.61