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Stock Based Compensation Share Based Compensation Shares Authorized under Stock Option Plans, Exercise Price Range (Details) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 269,589 470,356 622,945 738,792
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term 2 years 2 months 3 years 1 month 4 years 1 month 4 years 11 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 18.48 $ 18.57 $ 18.55 $ 18.39
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number 164,589 316,356 422,945  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price $ 18.48 $ 18.48 $ 18.48  
Eighteen​Point​Four​Eight​Exercise​Price [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
Investment Options, Exercise Price $ 18.48      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 269,589      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term 2 years 2 months      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 18.48      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number 164,589      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price $ 18.48      
PerformanceStock​Plan [Member]        
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]        
ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsOutstandingWeightedAverageGrantDateFairValue 9.03 8.71 8.62 8.72