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STOCK-BASED COMPENSATION - ACTIVITY (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
May. 31, 2014
May. 31, 2004
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Shares          
Options exercised (in shares)     (1,141,345) (1,515,023)  
Stock options          
Stock-based compensation          
Weighted-average grant date fair value of awards granted (in dollars per share)     $ 7.44 $ 4.83 $ 3.35
Total intrinsic value of options exercised     $ 18,500 $ 12,100 $ 1,700
Total fair value of options vested     7,800 6,200 5,100
Total unrecognized compensation expense, net of estimated forfeitures     $ 12,800    
Average vesting period for recognition of unrecognized compensation expense     1 year 10 months 24 days    
Cash received from stock option exercises     $ 7,900 $ 8,400 $ 2,800
Stock options | The 2004 Plan          
Stock-based compensation          
Number of shares authorized under plan   14,450,000      
Vesting period   4 years      
Shares          
Options outstanding at the beginning of the period (in shares)     6,002,334    
Options exercised (in shares)     (1,098,790)    
Options forfeited (in shares)     (201,392)    
Options expired (in shares)     (3,354)    
Options outstanding at the end of the period (in shares)     4,698,798 6,002,334  
Options vested and expected to vest (in shares)     4,577,274    
Options exercisable (in shares)     3,536,763    
Weighted-Average Exercise Price          
Options outstanding at the beginning of the period (in dollars per share)     $ 7.84    
Options exercised (in dollars per share)     6.68    
Options forfeited (in dollars per share)     9.43    
Options expired (in dollars per share)     6.66    
Options outstanding at the end of the period (in dollars per share)     8.05 $ 7.84  
Options vested and expected to vest (in dollars per share)     7.96    
Options exercisable (in dollars per share)     $ 7.32    
Weighted-Average Remaining Contractual Term          
Options outstanding at the end of the period     6 years 5 months 16 days    
Options vested and expected to vest     6 years 5 months 1 day    
Options exercisable at the end of the period     6 years 22 days    
Aggregate Intrinsic Value          
Options outstanding at the end of the period     $ 47,370    
Options vested and expected to vest     46,561    
Options exercisable at the end of the period     $ 38,241    
Stock options | The 2004 Plan | Minimum          
Stock-based compensation          
Awards vesting by the end of the first year (as a percent)   25.00%      
Stock options | The 2004 Plan | Maximum          
Stock-based compensation          
Term of awards   10 years      
Stock options | The 2014 Plan          
Stock-based compensation          
Number of shares authorized under plan 6,150,000        
Shares available for future issuance     2,759,767    
Vesting period 4 years        
Shares          
Options outstanding at the beginning of the period (in shares)     745,672    
Options granted (in shares)     1,990,025    
Options exercised (in shares)     (42,555)    
Options forfeited (in shares)     (169,521)    
Options expired (in shares)     (1,396)    
Options outstanding at the end of the period (in shares)     2,522,225 745,672  
Options vested and expected to vest (in shares)     2,051,327    
Options exercisable (in shares)     527,313    
Weighted-Average Exercise Price          
Options outstanding at the beginning of the period (in dollars per share)     $ 13.42    
Options granted (in dollars per share)     19.09    
Options exercised (in dollars per share)     12.12    
Options forfeited (in dollars per share)     18.16    
Options expired (in dollars per share)     14.77    
Options outstanding at the end of the period (in dollars per share)     17.59 $ 13.42  
Options vested and expected to vest (in dollars per share)     17.37    
Options exercisable (in dollars per share)     $ 15.96    
Weighted-Average Remaining Contractual Term          
Options outstanding at the end of the period     8 years 11 months 12 days    
Options vested and expected to vest     8 years 10 months 28 days    
Options exercisable at the end of the period     8 years 7 months 21 days    
Aggregate Intrinsic Value          
Options outstanding at the end of the period     $ 3,640    
Options vested and expected to vest     3,211    
Options exercisable at the end of the period     $ 1,220    
Stock options | The 2014 Plan | Minimum          
Stock-based compensation          
Awards vesting by the end of the first year (as a percent) 25.00%        
Stock options | The 2014 Plan | Maximum          
Stock-based compensation          
Term of awards 10 years        
Incentive stock options | The 2004 Plan | Minimum          
Stock-based compensation          
Exercise price of awards as percentage of the fair value of common stock 100.00%        
Nonstatutory stock options | The 2004 Plan | Minimum          
Stock-based compensation          
Exercise price of awards as percentage of the fair value of common stock 85.00%        
Restricted stock units | The 2004 Plan          
Stock-based compensation          
Vesting period     4 years    
Awards vesting by the end of the first year (as a percent)     25.00%    
Restricted stock units | The 2014 Plan          
Stock-based compensation          
Weighted-average grant date fair value of stock purchase rights granted (in dollars per share)     $ 18.27    
Vesting period     4 years    
Awards vesting by the end of the first year (as a percent)     25.00%    
ESPP          
Stock-based compensation          
Weighted-average grant date fair value of stock purchase rights granted (in dollars per share)     $ 6.69 $ 4.05 $ 1.87
Number of shares authorized under plan     2,500,000    
Shares available for future issuance     371,882    
ESPP | Minimum          
Stock-based compensation          
Exercise price of awards as percentage of the fair value of common stock   85.00%