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Restricted Stock Units - Narrative (Details) - USD ($)
shares in Thousands, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Vested deferred restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vested and remaining outstanding (in shares) 988 517  
Unvested units outstanding (in shares) 990   1,752
Unrecognized stock-based compensation expense $ 419    
Vested deferred restricted stock units | Non-Employee Directors      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vested and remaining outstanding (in shares) 988    
Performance-based Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unvested units outstanding (in shares) 546    
Unrecognized stock-based compensation expense $ 301    
Time-based Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unvested units outstanding (in shares) 443    
Unrecognized stock-based compensation expense $ 118    
Weighted average period for amortization of unrecognized stock-based compensation 5 months 20 days