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Equity-Based Compensation - Stock Option Activity (Detail)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Options outstanding, Beginning balance | shares 1,199,540 974,566  
Options granted | shares 605,200 374,700  
Options exercised | shares (48,621) (20,989)  
Options cancelled | shares (322,589) (128,737)  
Options outstanding, Ending balance | shares 1,433,530 1,199,540 974,566
Options exercisable, Ending balance | shares 529,639 534,164  
Options outstanding, Weighted Average Exercise Price, Beginning balance | $ / shares $ 44.64 $ 47.89  
Options granted, Weighted Average Exercise Price | $ / shares 28.50 37.54  
Options exercised, Weighted Average Exercise Price | $ / shares 21.14 17.83  
Options cancelled, Weighted Average Exercise Price | $ / shares 41.29 50.83  
Options outstanding, Weighted Average Exercise Price, Ending balance | $ / shares 39.35 44.64 $ 47.89
Options exercisable, Weighted Average Exercise Price, Ending balance | $ / shares $ 47.97 $ 44.98  
Options outstanding, Weighted Average Remaining Contractual Term 7 years 9 months 21 days 7 years 3 months 3 days 7 years 5 months 15 days
Options granted, Weighted Average Remaining Contractual Term 9 years 5 months 19 days 9 years 2 months 15 days  
Options exercisable, Weighted Average Remaining Contractual Term, Ending balance 6 years 7 days 5 years 8 months 23 days  
Options outstanding, Aggregate Intrinsic Value | $ $ 1,731 $ 2,717 $ 3,802
Options granted, Aggregate Intrinsic Value | $ 1,213 246  
Options exercised, Aggregate Intrinsic Value | $ 496 383  
Options cancelled, Aggregate Intrinsic Value | $ 0 0  
Options exercisable, Aggregate Intrinsic Value, Ending balance | $ $ 658 $ 2,386