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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]
9 Months Ended
Sep. 30, 2018
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Class A Common Stock, Beginning Balance | shares 1,484,175
Restricted Class A Common Stock, Granted | shares 318,741
Restricted Class A Common Stock, Vested | shares (404,241)
Restricted Class A Common Stock, Forfeited | shares (17,820)
Restricted Class A Common Stock, Ending Balance | shares 1,380,855
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance | $ / shares $ 30.61
Weighted-Average Grant Date Fair Value Per Share, Granted | $ / shares 31.83
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares 28.89
Weighted-Average Grant Date Fair Value Per Share, Forfeited | $ / shares 30.44
Weighted-Average Grant Date Fair Value Per Share, Ending Balance | $ / shares $ 31.40