Derivative Instruments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative [Line Items]
Loss to be reclassified from accumulated other comprehensive loss to interest expense over next twelve months	$ 3,100,000		$ 3,100,000
Interest Rate Swap [Member]
Derivative [Line Items]
Notional Amount	300,000,000		300,000,000
Interest Rate Swap 1 [Member]
Derivative [Line Items]
Notional Amount	125,000,000		125,000,000
Interest Rate Swap 2 [Member]
Derivative [Line Items]
Notional Amount	75,000,000		75,000,000
Interest Rate Swap 3 [Member]
Derivative [Line Items]
Notional Amount	50,000,000		50,000,000
Interest Rate Swap 4 [Member]
Derivative [Line Items]
Notional Amount	50,000,000		50,000,000
Credit Risk Contract [Member]
Derivative [Line Items]
Assets Needed for Immediate Settlement, Aggregate Fair Value	9,200,000		9,200,000
Interest Expense [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Amount of loss recognized in OCI on derivative	2,756,000	0	8,628,000	(204,000)
Amount of previously recorded loss reclassified from accumulated OCI into interest expense	(762,000)	(44,000)	(2,249,000)	(895,000)
Unsecured Debt [Member] | $300 Million Unsecured Term Loan [Member]
Derivative [Line Items]
Face amount of debt instument	$ 300,000,000		$ 300,000,000
Unsecured Debt [Member] | $300 Million Unsecured Term Loan [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Effective interest rate	2.69%		2.69%