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Commitments and Contingencies: (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]      
Ground rent expenses $ 11,870 $ 10,968 $ 10,579
Minimum future rental payments      
2016 15,695    
2017 15,632    
2018 11,249    
2019 9,629    
2020 9,637    
Thereafter 283,154    
Total 344,996    
Contingent Liabilities      
Contingent liability under letters of credit 62,788    
Outstanding obligations under construction agreements $ 32,006