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Incentive Plan (Long-Term Incentive Units) (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Long-term incentive units granted 159,220    
Long-Term Incentive Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Long-term incentive units granted 72,377 78,903  
Share/unit compensation expense $ 2.2 $ 1.9  
Service conditions of the awards The service conditions of the awards provide for 20% vesting on each of the first and second anniversaries of the grant date and 30% vesting on each of the third and fourth anniversaries of the grant date, provided the grantee continues employment on each anniversary date.    
Capitalized expense related to construction and leasing activities 0.2 0.2  
Unearned compensation $ 21.3   $ 12.7
Unearned compensation, period of recognition 3 years 1 month 6 days    
Vesting percentage, first and second anniversaries 20.00%    
Vesting percentage, third and fourth anniversaries 30.00%    
Service And Performance Condition [Member] | Long-Term Incentive Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Long-term incentive units granted 86,843 98,632  
Minimum [Member] | Long-Term Incentive Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Maximum [Member] | Long-Term Incentive Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 5 years