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Equity-Based Compensation - Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member] - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Unvested, Number of Shares/Units, Beginning balance 484,111 360,909
Granted, Number of Units 234,408 285,358
Cancelled, Number of Units (267,161) (89,173)
Vested, Number of Units   (72,983)
Unvested, Number of Shares/Units, Ending balance 451,358 484,111
Unvested, Weighted Average Grant-Date Fair Value, Beginning balance $ 44.52 $ 50.04
Granted, Weighted Average Grant-Date Fair Value 34.54 42.26
Cancelled, Weighted Average Grant-Date Fair Value 45.20 55.44
Vested, Weighted Average Grant-Date Fair Value   49.64
Unvested, Weighted Average Grant-Date Fair Value, Ending balance $ 38.94 $ 44.52