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Share-Based Compensation - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended 36 Months Ended
Jan. 01, 2019
Jan. 01, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value     $ 1,600 $ 3,800    
Shares authorized for issuance (in shares)     4,182,404   4,182,404  
Additional shares authorized for plan (in shares)   902,720        
Outstanding, exercisable equity awards     1,887,617   1,887,617  
Remaining equity awards available for future issuances     705,317   705,317  
Expiration period     10 years      
Percent of common stock authorized under plan     4.00%      
Unrecognized employee stock-based compensation costs     $ 8,300   $ 8,300  
Remaining weighted-average period for RSUs     2 years 7 months 1 day      
Proceeds from the exercise of stock options     $ 182 $ 307    
Options to purchase Common Stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares outstanding (in shares)     3,527,000 2,863,000 3,527,000 2,321,000
Expiration period     10 years      
Weighted average fair value per option (in usd per share)     $ 5.41 $ 8.27    
Shares issued in period (in shares)     1,108,000 989,000    
Options to purchase Common Stock | Initial vesting period            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percent     25.00%      
Options to purchase Common Stock | Subsequent vesting period            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting percent     75.00%      
Vesting period     36 months      
Options to purchase Common Stock | Maximum            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Vesting period     48 months      
Non-employee Stock Option | Non-employees            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares issued in period (in shares)     0      
2008 Equity Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Shares outstanding (in shares)     1,639,796   1,639,796  
2016 Equity Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Basis for equity incentive plan (in shares)     1,681,294   1,681,294  
Common stock            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Number of shares available for issuance under ESPP (in shares)     400,000   400,000  
Issuance of common stock for cash under employee stock purchase plan (in shares)     52,444 13,193 66,000  
Subsequent Event            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Additional shares authorized for plan (in shares) 1,233,578