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Commitments and Contingencies: (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Commitments and Contingencies:          
Ground rent expenses $ 2,244 $ 2,091 $ 6,329 $ 6,475  
Contingent Liabilities          
Contingent liability under letters of credit 4,224   4,224   19,721
Outstanding obligations under construction agreements $ 66,310   $ 66,310