Annual report pursuant to Section 13 and 15(d)

Stock Incentive Plan - Fair Value Assumptions (Details)

v3.8.0.1
Stock Incentive Plan - Fair Value Assumptions (Details) - Stock Options - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair value significant assumptions:      
Expected dividend yield $ 0 $ 0 $ 0
Risk-free interest rate, minimum 1.96% 1.16% 1.53%
Risk-free interest rate, maximum 2.36% 1.77% 2.01%
Expected volatility 46.00%    
Expected volatility, minimum   46.00% 44.00%
Expected volatility, maximum 49.00% 49.00% 49.00%
Expected life 6 years 6 months    
Minimum      
Fair value significant assumptions:      
Expected life   6 years 5 years
Maximum      
Fair value significant assumptions:      
Expected life   10 years 6 years 6 months