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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
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) 1,040,569 1,157,659 1,406,550
Granted (in shares) 243,940 314,060 390,200
Converted (in shares) (320,810) (329,200) (498,491)
Expired (in shares) (82,050) (92,750) (126,800)
Forfeited (in shares) (7,800) (9,200) (13,800)
Outstanding at end of period (in shares) 873,849 1,040,569 1,157,659
Vested (in shares) 441,424 472,294 511,109
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) $ 9.97 $ 7.52 $ 6.21
Granted, Weighted average grant-date fair value (in dollars per share) 17.52 15.47 9.61
Converted, Weighted average grant-date fair value (in dollars per share) 9.47 7.32 5.97
Expired, Weighted average grant-date fair value (in dollars per share) 9.22 7.44 5.52
Forfeited, Weighted average grant-date fair value (in dollars per share) 13.55 10.23 7.36
Outstanding at end of period, Weighted average grant-date fair value (in dollars per share) 12.30 9.97 7.52
Vested (in dollars per share) $ 7.61 $ 6.60 $ 6.03
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]      
Share-based compensation expense $ 3.6 $ 3.2 $ 2.5
Unrecognized share-based compensation expense $ 3.8    
Unrecognized share-based compensation, weighted average recognition period 1 year 6 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 1.70% 1.10% 0.90%
Expected volatility rate 34.70% 36.90% 41.50%
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) $ 20.18 $ 19.65 $ 10.99
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) $ 16.72 $ 14.39 $ 9.22
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) 1,084,524    
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) 324,599    
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