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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 6 Months Ended 12 Months Ended
Apr. 02, 2007
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Dec. 31, 2010
May 01, 2010
Vesting based on market conditions with respect to the first measurement date not achieved       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      
Common Stock, shares outstanding   98,754,727   98,754,727   91,159,221  
Class C Profit Units [Member]
             
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      
Class C units satisfying market conditions             613,485
Expected term (months)       36      
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            
Period of recognition in compensation expense (years) 5            
Unearned compensation due to the unvested portion of Class C Profits Interest Units       1.9   2.9  
Number of vested awards       497,023   439,653  
Stock option expense   0.5 0.5 1.1 0.9    
Capitalized expense related to construction and leasing activities   $ 0.1   $ 0.1