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Stock Incentive Plans - Summary of Stock Option Activity Under 2006 and 2016 Plans (Details) - 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 2,094,137 2,601,181  
Options granted 18,730 366,928  
Options exercised (203,552) (798,424)  
Options forfeited (23,401) (75,548)  
Options Outstanding, Ending balance 1,885,914 2,094,137 2,601,181
Options exercisable 1,230,759    
Options vested and expected to vest 1,850,457    
Options Outstanding, Weighted Average Exercise Price Per Share, Beginning Balance $ 23.20 $ 12.24  
Options Outstanding, Weighted Average Exercise Price Per Share, Options granted 83.54 68.32  
Options Outstanding, Weighted Average Exercise Price Per Share, Options exercised 10.42 7.19  
Options Outstanding, Weighted Average Exercise Price Per Share, Options forfeited 58.23 34.30  
Options Outstanding, Weighted Average Exercise Price Per Share, Ending Balance 24.74 $ 23.20 $ 12.24
Options Outstanding, Weighted Average Exercise Price Per Share, Exercisable 13.74    
Options Outstanding, Weighted Average Exercise Price Per Share, vested and expected to vest $ 24.18    
Options Outstanding, Weighted- Average Remaining Contractual Life (years) 6 years 11 months 15 days 7 years 7 days 7 years 2 months 1 day
Options exercisable, Weighted- Average Remaining Contractual Life (years) 6 years 3 months 18 days    
Options vested and expected to vest, Weighted- Average Remaining Contractual Life (years) 6 years 11 months 4 days    
Options Outstanding, Aggregate Intrinsic Value $ 95,499 $ 97,976 $ 113,958
Options exercisable, Aggregate Intrinsic Value 75,343    
Options vested and expected to vest, Aggregate Intrinsic Value $ 94,689