Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Summary of Significant Accounting Policies
Number of reportable segments			1
Derivative instruments
Net fair value of swap liability					$ 12,341,000
Cash Flow Hedging
Derivative instruments
Interest expenses	1,768,000	2,818,000	4,215,000	6,184,000
Interest rate contracts
Derivative instruments
Notional amount of outstanding derivative instruments	315,000,000		315,000,000
Net fair value of swap liability	11,100,000		11,100,000
Interest rate contracts | Cash Flow Hedging
Derivative instruments
Net fair value of swap liability	11,100,000		11,100,000		12,341,000
Interest expenses	$ 1,768,000	$ 2,818,000	$ 4,215,000	$ 6,184,000