Revolving Credit Facilities and Term Loan (Details) (USD $)				3 Months Ended					3 Months Ended
	Mar. 31, 2012	Jan. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Unsecured Revolving Credit Facility Arranged by JP Morgan Securities, LLC and Wells Fargo Securities, LLC [Member]	Mar. 31, 2012 Unsecured revolving credit facility with PNC Bank N.A. [Member]	Mar. 31, 2012 Secured Term Loan with Key Bank N A [Member]	Mar. 31, 2012 Unsecured Term Loan - Tranche 1 [Member]	Jan. 31, 2012 Unsecured Term Loan - Tranche 1 [Member]	Mar. 31, 2012 Unsecured Term Loan- Tranche 2 [Member]	Jan. 31, 2012 Unsecured Term Loan- Tranche 2 [Member]
Information Regarding the Company's Revolving Credit Facilities and Term Loans
Outstanding Balance				$ 76,000,000
Unsecured term loan	250,000,000	250,000,000					50,000,000	50,000,000	200,000,000	200,000,000
Secured Term Loan	$ 500,000,000		$ 500,000,000			$ 500,000,000
Weighted average interest rate				2.70%	1.90%
Maturity Date				February 2016	February 2016
Term loan weighted average interest rate						2.10%	1.90%		3.60%
Term loan maturity date						September 2014	January 2017		January 2019