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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 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) 276,496 206,695    
Converted (in shares) (130,764) (96,616)    
Expired (in shares) 0 0    
Forfeited (in shares) 0 0    
Outstanding at end of period (in shares) 1,115,660 983,928 969,928 873,849
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) 23.24 26.53    
Converted, Weighted average grant-date fair value (in dollars per share) 16.74 10.89    
Expired, Weighted average grant-date fair value (in dollars per share) 0.00 0.00    
Forfeited, Weighted average grant-date fair value (in dollars per share) 0.00 0.00    
Outstanding at end of period, Weighted average grant-date fair value (in dollars per share) 17.61 15.43 $ 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 $ 1.5 $ 1.0    
Unrecognized share-based compensation expense $ 12.1      
Unrecognized share-based compensation, weighted average recognition period 1 year 10 months      
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) 1,358,510      
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] | Minimum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite service period 6 months      
Director [Member] | Time-Based Awards [Member] | Maximum [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Requisite service period 12 months