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Summary of Significant Accounting Policies (Details 5) (USD $)
In Millions, except Per Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Stock-Based Compensation      
Total stock-based compensation pre-tax $ 7.9 $ 6.9 $ 6.3
Tax benefit 1.2 1.1 1.1
Total stock-based compensation net of tax 6.7 5.8 5.2
Estimated forfeiture rate (as a percent) 5.20% 5.40% 5.80%
Stock options
     
Stock-Based Compensation      
Total stock-based compensation pre-tax 6.6 5.8 5.0
Award vesting period, minimum (in years) P3Y    
Award vesting period, maximum (in years) P5Y    
Risk-free interest rate, minimum (as a percent) 1.24% 1.73% 1.71%
Risk-free interest rate, maximum (as a percent) 3.12% 3.46% 3.60%
Expected life (in years) 6.5 6.5 6.5
Volatility (as a percent) 57.20% 62.00% 64.00%
Weighted average fair values of options granted (in dollars per share) $ 7.89 $ 8.56 $ 5.83
Restricted stock
     
Stock-Based Compensation      
Total stock-based compensation pre-tax $ 1.3 $ 1.1 $ 1.3
Award vesting period, minimum (in years) P3Y    
Award vesting period, maximum (in years) P5Y