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Stock-Based Compensation (Details Textual) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Recognized / Unrecognized compensation cost $ 0.9    
Stock Based Compensation (Textual) [Abstract]      
Compensation expense related to Stock-based Plans 4.3 1.5 0.2
Safety Incentive Plan [Member]
     
Stock Based Compensation (Textual) [Abstract]      
Compensation expense related to Stock-based Plans 0.7 0.6 0.6
1995 incentive plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Forfeiture restrictions percentage 100.00%    
Vesting period of employees 3 years    
Weighted average grant date fair value $ 20.42    
Aggregate value of vested shares 1.7    
2012 Long - Term Incentive Plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Voting shares available for issuance 560,433    
Forfeiture restrictions percentage 100.00%    
Vesting period of employees 3 years    
Non-voting common stock [Member] | 1995 incentive plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Non-voting authorized to issue 575,000    
Voting shares available for issuance 738    
Non-voting common stock [Member] | 2012 Long - Term Incentive Plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Non-voting authorized to issue 750,000    
Voting common stock [Member] | 1995 incentive plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Non-voting authorized to issue 175,000    
Voting shares available for issuance 32,542    
Non-Voting Time-Vested Restricted Stock Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value   $ 19.36 $ 21.05
Aggregate value of vested shares 3.2 0 0
Weighted average period 2 years 2 months 9 days    
Non-Voting Performance Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average period 2 years 2 months 12 days    
Recognized / Unrecognized compensation cost $ 2.6    
Voting Restricted Stock Units [Member] | 1995 incentive plan [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Voting common stock outstanding under the 1995 Incentive Plan 83,978