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Stock-Based Compensation - Performance Share Units (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Jan. 01, 2018
$ / shares
shares
Jan. 01, 2017
$ / shares
shares
Jan. 01, 2016
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Stock-based compensation            
Stock-based compensation expense | $       $ 6.9 $ 3.1 $ 4.5
Employee stock-based compensation plan            
Stock-based compensation            
Total shares authorized       5,600,000    
Shares available for future grants       4,700,000    
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) 506,051 321,612   506,051 321,612  
Granted (in shares)       277,197 244,218 321,612
Vested (in shares)       (229,809) (38,522)  
Forfeited (in shares)       (48,942) (21,257)  
Unvested as of end of period (in shares)       504,497 506,051 321,612
Weighted Average Grant Date Fair Value            
Unvested as of beginning of period (in dollars per share) | $ / shares $ 30.82 $ 29.11   $ 30.82 $ 29.11  
Granted (in dollars per share) | $ / shares       22.39 30.44 $ 29.11
Vested (in dollars per share) | $ / shares       32.17 24.41  
Forfeited (in dollars per share) | $ / shares       30.52 33.20  
Unvested as of end of period (in dollars per share) | $ / shares       $ 25.93 $ 30.82 $ 29.11
Stock-based compensation            
Stock-based compensation expense | $       $ 6.5 $ 2.9 $ 2.6
Unrecognized compensation expense | $       $ 6.2 4.3  
Period for recognition of unrecognized compensation expense       1 year 6 months    
Total fair value of vested shares (in dollars) | $       $ 7.4 $ 0.9  
Performance Share Units | Key employees            
Number of Shares            
Granted (in shares)       277,197 209,374 356,456
Weighted Average Grant Date Fair Value            
Granted (in dollars per share) | $ / shares           $ 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 3 years      
Long‑Term Incentive Plan | Salaries and employee benefits            
Stock-based compensation            
Stock-based compensation expense | $       $ 3.3 $ 6.7 $ 9.3
Long‑Term Incentive Plan | Performance Share Units            
Number of Shares            
Granted (in shares)           240,890