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Stock-Based Compensation Plans Stock Performance Rights (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock Performance Rights      
Accrued Employee Benefits, Current $ 8,712,000 $ 7,859,000  
Valuation assumptions:      
Allocated Share-based Compensation Expense $ 200,000    
Number of SPRs      
Exercised (40,000)    
Stock Performance Rights      
Stock Performance Rights      
Weighted average estimated value of SPRs outstanding (per share) $ 9.83    
Valuation assumptions:      
Expected volatility, minimum, percent 20.50%    
Expected volatility, maximum, percent 44.20%    
Risk-free rate of return, minimum 1.20%    
Risk-free rate of return, maximum 2.20%    
Expected annual dividend $ 0.00    
Allocated Share-based Compensation Expense $ 1,200,000 $ 1,200,000 $ 900,000
Number of SPRs      
Outstanding on December 31, 2016 946,701    
Granted 78,948    
Exercised (34,095)    
Cancelled (30,000)    
Outstanding on December 31, 2017 961,554 946,701  
Exercisable on December 31, 2017 702,438    
Weighted Average Exercise Price      
Outstanding on December 31, 2016 $ 19.60    
Granted 25.12    
Exercised 12.99    
Cancelled 36.71    
Outstanding on December 31, 2017 9.76 $ 19.60  
Exercisable on December 31, 2017 $ 17.66    
SPRs outstanding, intrinsic value $ 6,300,000    
Total unrecognized compensation cost $ 1,700,000    
Unrecognized cost, period for recognition 1 year 10 months 24 days    
Exercised 152,232    
Vested in period, fair value $ 800,000    
Weighted average remaining contractual term, SPRs outstanding 4 years 8 months    
Weighted average remaining contractual term, SPRs exercisable 4 years 5 months    
Stock Performance Rights | Minimum      
Stock Performance Rights      
Life of award 7 years    
Award vesting period 1 year    
Valuation assumptions:      
Expected term 1 month 6 days    
Stock Performance Rights | Maximum      
Stock Performance Rights      
Life of award 10 years    
Award vesting period 3 years    
Valuation assumptions:      
Expected term 4 years 6 months