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Incentive Plan (Restricted Stock) (Narrative) (Details) (Restricted Stock [Member], USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
May 01, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted shares granted 42,720 40,807    
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.      
Restricted stock expense $ 0.7 $ 0.4    
Capitalized expense related to construction and leasing activities 0.4 0.3    
Unearned compensation $ 11.1   $ 5.5  
Unearned compensation, period of recognition 3 years 3 months 18 days      
Awards satisfying market conditions on measurement date       20,169
Vesting percentage, first and second anniversaries 20.00%      
Vesting percentage, third and fourth anniversaries 30.00%      
Service And Performance Condition [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted shares granted 52,947 50,999