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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]
6 Months Ended
Jun. 30, 2019
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Class A Common Stock, Beginning Balance | shares 1,614,907
Restricted Class A Common Stock, Granted | shares 334,904
Restricted Class A Common Stock, Vested | shares (420,780)
Restricted Class A Common Stock, Forfeited | shares (17,565)
Restricted Class A Common Stock, Ending Balance | shares 1,511,466
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance | $ / shares $ 32.94
Weighted-Average Grant Date Fair Value Per Share, Granted | $ / shares 31.54
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares 32.15
Weighted-Average Grant Date Fair Value Per Share, Forfeited | $ / shares 31.55
Weighted-Average Grant Date Fair Value Per Share, Ending Balance | $ / shares $ 32.87