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Commitments and Contingencies (Details) (USD $)
12 Months Ended
Dec. 31, 2011
property
Dec. 31, 2010
Dec. 31, 2009
Commitments and Contingencies      
Properties subject to ground leases 32    
Ground lease expense $ 42,284,000 $ 36,750,000 $ 32,086,000
Operating Leases, Future Minimum Payments Due      
2012 26,193,000    
2013 26,627,000    
2014 26,593,000    
2015 27,272,000    
2016 27,442,000    
Thereafter 851,184,000    
Total 985,311,000    
Insurance      
Insurance coverage, acts of terrorism $ 1,000,000,000