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Stock-Based Compensation - Restricted Stock (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Restricted Stock    
Number of Shares    
Unvested as of beginning of period (in shares)  
Granted (in shares) 0 77,037
Vested (in shares)   (77,037)
Unvested as of end of period (in shares)  
Weighted Average Grant Date Fair Value    
Unvested as of beginning of period (in dollars per share)  
Granted (in dollars per share)   $ 24.41
Vested (in dollars per share)   24.41
Unvested as of end of period (in dollars per share)  
Stock-based compensation    
Total fair value of vested shares (in dollars)   $ 1.9
Restricted stock, tranche one    
Stock-based compensation    
Percentage of vested award with transfer restrictions lapsing after a specified holding period   50.00%
Period following vesting date for lapsing of transfer restrictions   6 months
Restricted stock, tranche two    
Stock-based compensation    
Percentage of vested award with transfer restrictions lapsing after a specified holding period   50.00%
Period following vesting date for lapsing of transfer restrictions   18 months