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Stockholders' Equity - Schedule of Share Based Compensation Restricted Stock Units Award Activity (Detail) - Restricted Stock Units (ZSUs) [Member] - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Unvested Outstanding Shares, Beginning balance 52,482  
Unvested Shares, Granted 3,503  
Unvested Shares, Vested (5,717)  
Unvested Shares, Forfeited (1,051)  
Unvested Outstanding Shares, Ending balance 49,217  
Unvested Weighted Average Grant Date Fair Value, Beginning balance $ 3.49  
Unvested Weighted Average Grant Date Fair Value, Granted 5.18  
Unvested Weighted Average Grant Date Fair Value, Vested 3.24  
Unvested Weighted Average Grant Date Fair Value, Forfeited 2.77  
Unvested Weighted Average Grant Date Fair Value, Ending balance $ 3.66  
Unvested, Aggregate Intrinsic Value of Unvested ZSU $ 262,332 $ 206,254