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Incentive Plan (Class C Profits Interest Units) (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2012
May 01, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common Stock, shares outstanding 128,413,791   125,140,783  
Class C Profit Units [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Awards satisfying market conditions on measurement date       593,316
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% vested ratably each month through June 30, 2012.      
Award vesting percentage, vested       60.00%
Award vesting percentage, unvested       40.00%
Approximate number of times simulation was run 100,000      
Expected term 36 months      
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      
Requisite service period of share-based award 5 years      
Restricted Stock [Member]
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Awards satisfying market conditions on measurement date       20,169
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 provided the grantee continues employment on each anniversary date.      
Unearned compensation 13.4   7.4  
Capitalized expense related to construction and leasing activities $ 0.6 $ 0.4