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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, 2015
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Class A Common Stock, Beginning Balance 919,719
Restricted Class A Common Stock, Granted 190,674
Restricted Class A Common Stock, Vested (342,978)
Restricted Class A Common Stock, Forfeited (28,143)
Restricted Class A Common Stock, Ending Balance 739,272
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance | $ / shares $ 26.86
Weighted-Average Grant Date Fair Value Per Share, Granted | $ / shares 29.39
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares 26.94
Weighted-Average Grant Date Fair Value Per Share, Forfeited | $ / shares 28.27
Weighted-Average Grant Date Fair Value Per Share, Ending Balance | $ / shares $ 27.42