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Incentive Plan (Class C Profits Interest Units) (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
0 Months Ended 3 Months Ended 12 Months Ended
Apr. 02, 2007
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
May 01, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Common Stock, shares outstanding   107,342,049   106,039,279  
Class C Profit Units [Member]
         
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting based on market conditions with respect to the first measurement date not achieved   Of the Class C Units that satisfied the market condition on May 1, 2010, 60% vested on May 1, 2010 and the remaining 40% will vest ratably each month thereafter for 24 months.      
Awards satisfying market conditions on measurement date         593,316
Expected term   3 years      
Expected volatility rate   23.00%      
Risk-free interest rate   4.60%      
Expected dividend rate   5.00%      
Award limit, first market condition   $ 17      
Award limit, second market condition   40      
Common Stock, shares outstanding   69,200,000      
Grant date fair value of awards 11.8        
Unearned compensation   0.2   0.6  
Requisite service period of share-based award   5 years      
Number of vested awards   580,554   558,872  
Stock option expense   0.4 0.5    
Capitalized expense related to construction and leasing activities   $ 0.1 $ 0.1    
Award vesting percentage, vested         60.00%
Award vesting percentage, unvested         40.00%