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Equity Incentive Compensation Plans (Restricted Stock Units and Performance Share Program Parenthetical) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
0 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2012
Mar. 31, 2012
simulation
Mar. 31, 2011
simulation
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Equity Incentive Compensation Plans            
Total unrecognized compensation cost $ 2.5     $ 2.5    
Period of recognition of unrecognized compensation cost (in years) 3 years 3 months          
Vesting period (in years)       4 years    
Fair value assumptions used in market-based portion of performance shares valuation model (Monte Carlo model)            
Expected volatility (as a percent)       50.00% 45.00%  
Risk-free interest rate (as a percent)       0.95% 2.54%  
Performance Based Restricted Stock Units
           
Equity Incentive Compensation Plans            
Total unrecognized compensation cost 2.1     2.1    
Period of recognition of unrecognized compensation cost (in years) 2 years          
Granted (in shares)       59,738 65,620 103,794
Number of years of service to be completed to participate in the program (in years)       3 years    
Minimum interest coverage ratio, numerator       2.5    
Minimum interest coverage ratio, denominator       1    
Vesting period (in years)       3 years    
Grant date fair value determined by Monte Carlo valuation model   1.3 1.1      
Fair value assumptions used in market-based portion of performance shares valuation model (Monte Carlo model)            
Number of simulations   100,000 100,000      
Expected volatility (as a percent)   50.00% 44.00%      
Risk-free interest rate (as a percent)   0.42% 1.15%      
Restricted Stock Units
           
Equity Incentive Compensation Plans            
Total unrecognized compensation cost $ 11.1     $ 11.1    
Period of recognition of unrecognized compensation cost (in years) 3 years 3 months          
Granted (in shares)       164,112 159,333 34,529
Restricted Stock Units | Executive officers and other officers
           
Equity Incentive Compensation Plans            
Fully Vested Shares Outstanding Subject to Deferral Election by Executive Officers or Other Officers (in shares) 614,996     614,996 483,908 289,335
Restricted Stock Units | Non-employee directors
           
Equity Incentive Compensation Plans            
Percentage of stock units vested at date of grant (as a percent) 100.00%     100.00%    
Fully Vested Shares Outstanding Subject to Non-Employee Director Deferral Election (in shares) 43,554     43,554 30,544 10,522