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Stock Compensation - Additional Disclosures (Details) - USD ($)
$ / shares in Units, $ in Millions
1 Months Ended 2 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Mar. 31, 2015
Feb. 28, 2015
Dec. 31, 2017
Dec. 31, 2016
Sep. 30, 2018
Sep. 30, 2016
Oct. 01, 2017
Oct. 01, 2016
May 01, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Share price (usd per share)                   $ 9.50    
2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Additional shares authorized (shares)       1,081,200 643,673              
Compensation not yet recognized     $ 1.5                  
Common Stock Class A Non-Voting | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares authorized (shares)                       5,085,649
Increase (decrease) of shares authorized for grant (shares)   500,000 185,026     1,100,000            
Restricted Stock and Phantom Share-Based Awards                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Unamortized fair value of awards               $ 9.0        
Weighted average period over which costs amortized               2 years        
Restricted stock | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Options granted, weighted average grant date fair value (usd per share) $ 9.75   $ 2.96       $ 9.60          
Share price (usd per share)                     $ 11.06  
Options forfeited in period (shares)     720,000                  
Options forfeited in period, grant date fair value     $ 3.4                  
Restricted stock | Employee | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Options granted in period (shares) 1,308,533   961,718       931,260          
Restricted stock | Non-employee directors                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Award vesting period               2 years        
Achievement performance target, probable percentage               80.00%        
Restricted stock | Non-employee directors | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Options granted in period (shares) 84,250   130,000       72,500          
Options granted, weighted average grant date fair value (usd per share)     $ 6.17                  
Performance based awards | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Options granted in period (shares)     421,394                  
Compensation not yet recognized     $ 0.8                  
Vesting Tranche 1 | Restricted stock | Employee | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Options granted in period (shares) 190,725                      
Vesting Tranche 1 | Restricted stock | Non-employee directors                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Vesting rights percentage               50.00% 50.00%      
Vesting Tranche 2 | Restricted stock | Employee | 2010 Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Options granted in period (shares) 1,117,808                      
Vesting Tranche 2 | Restricted stock | Non-employee directors                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Vesting rights percentage               50.00%        
Vesting Tranche 3 | Restricted stock | Non-employee directors                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Vesting rights percentage               20.00%