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Equity-Based Compensation - Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Options outstanding, Beginning balance 1,199,540 974,566  
Options granted 566,800 374,700  
Options exercised (18,315) (20,989)  
Options cancelled (118,707) (128,737)  
Options outstanding, Ending balance 1,629,318 1,199,540 974,566
Options exercisable, Ending balance 611,099 534,164  
Options outstanding, Weighted Average Exercise Price, Beginning balance $ 44.64 $ 47.89  
Options granted, Weighted Average Exercise Price 28.25 37.54  
Options exercised, Weighted Average Exercise Price 15.96 17.83  
Options cancelled, Weighted Average Exercise Price 42.43 50.83  
Options outstanding, Weighted Average Exercise Price, Ending balance 39.39 44.64 $ 47.89
Options exercisable, Weighted Average Exercise Price, Ending balance $ 46.73 $ 44.98  
Options outstanding, Weighted Average Remaining Contractual Term 8 years 1 month 20 days 7 years 3 months 3 days 7 years 5 months 15 days
Options granted, Weighted Average Remaining Contractual Term 9 years 11 months 8 days 9 years 2 months 15 days  
Options exercisable, Weighted Average Remaining Contractual Term, Ending balance 6 years 3 months 14 days 5 years 8 months 23 days  
Options outstanding, Aggregate Intrinsic Value $ 762 $ 2,717 $ 3,802
Options granted, Aggregate Intrinsic Value 45 246  
Options exercised, Aggregate Intrinsic Value 225 383  
Options cancelled, Aggregate Intrinsic Value 0 0  
Options exercisable, Aggregate Intrinsic Value, Ending balance $ 717 $ 2,386