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Share-Based Compensation Performance shares (Details) (Performance Shares [Member], USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Transactions under the terms of the LTSIP - shares [Roll Forward]    
Balance at December 31, 2012 264,867  
Granted 126,606  
Balance at March 31, 2013 391,473  
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at December 31, 2012 $ 21.94  
Granted $ 39.62  
Balance at March 31, 2013 $ 27.66  
Share-based awards other than options, additional disclosures [Abstract]    
Performance period that the target number of performance shares for each exectutive officer is subject to adjustment based on the company's performance (in years) 3 years  
Equity- and liability-based performance share compensation expense $ 0.8 $ 0.4
Weighted-average remaining vesting period of share-based awards (in months) 21 months  
Minimum [Member]
   
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at December 31, 2012 $ 18.23  
Granted     
Balance at March 31, 2013 $ 18.23  
Maximum [Member]
   
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at December 31, 2012 $ 25.42  
Granted $ 39.62  
Balance at March 31, 2013 $ 39.62