| 
           Share-Based Compensation (Details) (USD $) 
        In Millions, unless otherwise specified  | 
        3 Months Ended | 9 Months Ended | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 
           Sep. 30, 2012 
         | 
        
           Sep. 30, 2011 
         | 
        
           Sep. 30, 2012 
         | 
        
           Sep. 30, 2011 
         | 
        
           Sep. 30, 2012
 
          1997 Plan, 2003 Plan and 2011 Plan [Member]
 
          Y 
         | 
        
           Dec. 31, 2011
 
          1997 Plan, 2003 Plan and 2011 Plan [Member] 
         | 
        
           Jul. 31, 2012
 
          Cash Awards [Member] 
         | 
        
           Jul. 31, 2012
 
          Restricted Stock [Member] 
         | 
        
           Jul. 31, 2012
 
          Restricted Stock [Member]
 
          Issuance of NTP [Member] 
         | 
        
           Jul. 31, 2012
 
          Restricted Stock [Member]
 
          First Anniversary of NTP [Member] 
         | 
        
           Jul. 31, 2012
 
          Restricted Stock [Member]
 
          Second Anniversary of NTP [Member] 
         | 
        
           Jul. 31, 2012
 
          Restricted Stock [Member]
 
          Third Anniversary of NTP [Member] 
         | 
        
           Jul. 31, 2012
 
          Restricted Stock [Member]
 
          Fourth Anniversary of NTP [Member] 
         | 
      |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||||||||||||
| Share-Based Compensation Arrangement By Share-Based Payment Award, Value of Cash Awards Authorized | $ 60 | ||||||||||||
| Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | 10 | ||||||||||||
| Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Percentage | 35.00% | 10.00% | 15.00% | 15.00% | 25.00% | ||||||||
| Total share-based compensation expense recognized | 49.8 | 2.3 | 54.0 | 16.6 | |||||||||
| Total share-based compensation capitalized | 2.2 | ||||||||||||
| Total unrecognized compensation cost related to non-vested share-based compensation | 94.7 | 7.7 | |||||||||||
| Recognition period for total unrecognized compensation expense | 4 | ||||||||||||
| Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition | 3.8 | ||||||||||||
| Proceeds from the exercise of stock options | $ 0.8 | $ 0.8 | $ 0.8 | ||||||||||