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Derivative Instruments and Hedging Activities: (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
Derivative Instruments and Hedging Activities:        
Other comprehensive income related to marking-to-market of interest rate agreements $ 0 $ 1,358 $ 0 $ 6,188