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Fair Value Measurements - Rollforward of Level 3 financial instruments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2014
Dec. 31, 2015
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]    
Fair vale Level 1 and 2 transfers $ 0 $ 0
Fair vale Level 2 and 1 transfers 0 $ 0
Warrants to purchase convertible preferred stock    
Roll forward of the financial instruments classified within Level 3 of the fair value hierarchy    
Balance at the beginning of the period 6,232  
Change in fair value recorded in Other (income) expense, net 983  
Exercise of warrants $ (7,215)