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Share-Based Compensation Performance shares (Details) (Performance Shares [Member], USD $)
In Millions, except Share data, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Transactions under the terms of the LTSIP - shares [Roll Forward]    
Balance at December 31, 2012 264,867  
Granted 126,606  
Forfeited (3,594)  
Balance at September 30, 2013 387,879  
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at December 31, 2012 $ 21.94  
Granted $ 39.62  
Forfeited $ 33.09  
Balance at September 30, 2013 $ 27.61  
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 $ 1.2 $ 1.4
Weighted-average remaining vesting period of share-based awards (in months) 15 months  
Vested Undistributed [Member]
   
Transactions under the terms of the LTSIP - shares [Roll Forward]    
Balance at September 30, 2013 5,330  
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at September 30, 2013 $ 23.39  
Minimum [Member]
   
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at December 31, 2012 $ 18.23  
Granted $ 39.62  
Forfeited $ 18.23  
Balance at September 30, 2013 $ 18.23  
Minimum [Member] | Vested Undistributed [Member]
   
Share-based awards other than options, additional disclosures [Abstract]    
The date the actual performance shares (if any) will be distributed first quarter of 2014  
Maximum [Member]
   
Transactions under the terms of LTSIP - grant-date fair value [Roll Forward]    
Balance at December 31, 2012 $ 25.42  
Granted $ 39.62  
Forfeited $ 39.62  
Balance at September 30, 2013 $ 39.62  
Maximum [Member] | Vested Undistributed [Member]
   
Share-based awards other than options, additional disclosures [Abstract]    
The date the actual performance shares (if any) will be distributed first quarter of 2016