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Commitments and Contingencies: (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Dec. 31, 2011
Commitments and Contingencies:      
Ground rent expenses $ 2,093 $ 2,212  
Contingent Liabilities      
Contingent liability under letters of credit     19,721
Outstanding obligations under construction agreements $ 9,019