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Stock-Based Compensation - Performance Share Units (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Jan. 01, 2017
$ / shares
shares
Jan. 01, 2016
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Stock-based compensation        
Stock-based compensation expense | $     $ 3.1 $ 4.5
Employee stock-based compensation plan        
Stock-based compensation        
Total shares authorized     5,600,000  
Shares available for future grants     5,000,000  
Performance Share Units        
Stock-based compensation        
Ratio of common stock for each vested award     1  
Number of Shares        
Unvested as of beginning of period (in shares) 321,612   321,612  
Granted (in shares)     244,218 321,612
Vested (in shares)     (38,522)  
Forfeited (in shares)     (21,257)  
Unvested as of end of period (in shares)     506,051 321,612
Weighted Average Grant Date Fair Value        
Unvested as of beginning of period (in dollars per share) | $ / shares $ 29.11   $ 29.11  
Granted (in dollars per share) | $ / shares     30.44 $ 29.11
Vested (in dollars per share) | $ / shares     24.41  
Forfeited (in dollars per share) | $ / shares     33.20  
Unvested as of end of period (in dollars per share) | $ / shares     $ 30.82 $ 29.11
Stock-based compensation        
Stock-based compensation expense | $     $ 2.9 $ 2.6
Unrecognized compensation expense | $     $ 4.3 $ 6.8
Period for recognition of unrecognized compensation expense     1 year 6 months  
Total fair value of vested shares (in dollars) | $     $ 0.9  
Performance Share Units | Key employees        
Number of Shares        
Granted (in shares)     209,374 356,456
Weighted Average Grant Date Fair Value        
Granted (in dollars per share) | $ / shares     $ 29.98 $ 30.91
IPO | Performance Share Units        
Number of Shares        
Granted (in shares)       115,566
Stock-based compensation        
Period following vesting date for lapsing of transfer restrictions       6 months
IPO | Performance Share Units | Tranche one        
Stock-based compensation        
Vesting (as a percent)       33.33%
IPO | Performance Share Units | Tranche two        
Stock-based compensation        
Vesting (as a percent)       33.33%
IPO | Performance Share Units | Tranche three        
Stock-based compensation        
Vesting (as a percent)       33.33%
Long‑Term Incentive Plan        
Stock-based compensation        
Performance period 3 years 3 years    
Long‑Term Incentive Plan | Performance Share Units        
Number of Shares        
Granted (in shares)       240,890