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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Outstanding at beginning of period (in shares) 969,928 873,849 873,849  
Granted (in shares) 302,596 207,295    
Converted (in shares) (164,864) (96,616)    
Expired (in shares) (7,500) (500)    
Forfeited (in shares) (6,000) (2,600)    
Outstanding at end of period (in shares) 1,094,160 981,428 969,928 873,849
Vested (in shares) 337,060 326,928    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]        
Outstanding at beginning of period, Weighted average grant-date fair value (in dollars per share) $ 15.89 $ 12.30 $ 12.30  
Granted, Weighted average grant-date fair value (in dollars per share) 23.22 26.50    
Converted, Weighted average grant-date fair value (in dollars per share) 17.48 10.89    
Expired, Weighted average grant-date fair value (in dollars per share) 23.78 28.38    
Forfeited, Weighted average grant-date fair value (in dollars per share) 23.73 26.76    
Outstanding at end of period, Weighted average grant-date fair value (in dollars per share) 17.58 15.39 $ 15.89 $ 12.30
Vested (in dollars per share) $ 8.04 $ 7.18    
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Share-based compensation expense $ 5.3 $ 3.6    
Unrecognized share-based compensation expense $ 8.4      
Unrecognized share-based compensation, weighted average recognition period 1 year 3 months 18 days      
Restricted Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite service period 3 years      
Stock Units [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite service period 3 years      
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Historical volatility period 3 years   3 years 3 years
Risk-free interest rate 2.50%      
Expected volatility rate 35.60%      
Market Condition Award [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]        
Granted, Weighted average grant-date fair value (in dollars per share) $ 24.75      
Performance Condition Award [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]        
Granted, Weighted average grant-date fair value (in dollars per share) $ 22.80      
Time-Based Awards [Member] | Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]        
Number of additional outstanding shares issued (in shares) 1,329,135      
Director [Member] | Time-Based Awards [Member]        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]        
Vested (in shares) 337,060      
Director [Member] | Time-Based Awards [Member] | Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite service period 12 months