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Stockholders' Equity - Summary of MSU Performance (Detail) - Market Performance Based Restricted Stock Units - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Number of Shares    
Nonvested beginning balance (shares) 428,000  
Granted (shares) 208,000  
Vested and released (shares) (312,300)  
Forfeited (shares) 0  
Nonvested ending balance (shares) 324,000 428,000
Nonvested (usd per share) $ 215.07 $ 78.53
Granted (usd per share) 266.78  
Vested and released (usd per share) 62.41  
Forfeited (usd per share) $ 0.00  
Weighted Average Remaining Contractual Term    
Nonvested 1 year 1 month 28 days  
Aggregate Intrinsic Value    
Nonvested $ 67,897