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Incentive Plan (Long-Term Incentive Units) (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Long-term incentive units granted     192,242    
Long-Term Incentive Units [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Long-term incentive units granted     89,769 79,237  
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.    
Vesting percentage, first and second anniversaries     20.00%    
Vesting percentage, third and fourth anniversaries     30.00%    
Share/unit compensation expense $ 2.8 $ 2.9 $ 5.1 $ 5.1  
Capitalized expense related to construction and leasing activities 0.4 0.2 0.9 0.4  
Unearned compensation $ 18.8   $ 18.8   $ 13.3
Unearned compensation, period of recognition     2 years 10 months 24 days    
Service And Performance Condition [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Long-term incentive units granted     95,316 86,843  
Service And Performance Condition [Member] | Long-Term Incentive Units [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of shares satisfying the performance conditions     78,118    
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