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Employee Benefit Plans - Valuation Disclosures (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Options      
Dividend yield 0.00%   0.00%
Stock Options | Minimum      
Share price $ 2.27 $ 2.06 $ 2.50
Expected life (in years) 4 years 5 months 9 days 4 years 4 months 21 days 4 years 4 months 24 days
Interest rate 1.09% 1.30% 1.30%
Volatility 52.40% 53.50% 51.40%
Stock Options | Maximum      
Share price $ 5.30 $ 2.80 $ 3.49
Expected life (in years) 4 years 5 months 16 days 4 years 4 months 28 days 4 years 5 months 19 days
Interest rate 1.58% 1.60% 1.50%
Volatility 54.00% 54.60% 52.50%
ESPP shares      
Expected life (in years) 1 year 6 months   1 year 6 months
Dividend yield 0.00%   0.00%
ESPP shares | Minimum      
Share price $ 2.58 $ 2.15 $ 2.55
Interest rate 0.30% 0.10% 0.10%
Volatility 42.60% 46.90% 47.30%
ESPP shares | Maximum      
Share price $ 4.97 $ 3.00 $ 4.70
Interest rate 0.90% 0.50% 0.30%
Volatility 46.20% 55.90% 55.60%