Quarterly report pursuant to Section 13 or 15(d)

Noncontrolling Interests In Subsidiaries (Narrative) (Details)

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Noncontrolling Interests In Subsidiaries (Narrative) (Details) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Noncontrolling Interest [Line Items]    
Number of common shares received upon redemption of common units 1  
Rebalance of ownership percentage $ 0.7  
Percentage of noncontrolling interest 12.10% 12.20%
Participation Rights [Member]
   
Noncontrolling Interest [Line Items]    
Excess net cash flow remaining after the distribution to the Company 50.00%  
Internal rate of return 10.00%  
Future Developments [Member] | Participation Rights [Member]
   
Noncontrolling Interest [Line Items]    
Number of properties 2  
Office Buildings [Member] | Participation Rights [Member]
   
Noncontrolling Interest [Line Items]    
Number of properties 3  
Rentable Square Feet 760,236