Derivative Instruments and Hedging Activities (Schedule of Notional Values and Fair Values of Derivative Instruments) (Details) (Cash Flow Interest Rate Contracts [Member], Designated as Hedging Instrument [Member], USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Notional Value	$ 768,269	$ 491,807
Other Assets [Member]
Asset derivatives, Positive fair value	0	0
Accounts Payable and Accrued Expenses [Member]
Liability derivatives, Negative fair value	$ (11,857)	$ (9,259)