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Commitments and Contingencies: (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Commitments and Contingencies:          
Ground rent expenses $ 1,992 $ 2,172 $ 4,086 $ 4,384  
Contingent Liabilities          
Contingent liability under letters of credit 4,574   4,574   19,721
Outstanding obligations under construction agreements $ 92,060   $ 92,060