Secured and unsecured senior debt (Details 4) (USD $)	3 Months Ended		6 Months Ended								1 Months Ended	6 Months Ended		1 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Apr. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans	Jun. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Less than or equal to	Jun. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Greater than or equal to	Jun. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Requirement Less than or equal to	Jun. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Requirement Greater than or equal to	Apr. 30, 2012 Unsecured Credit Facility line of credit extension	Jun. 30, 2012 Unsecured Credit Facility line of credit	Apr. 30, 2012 Unsecured senior line of credit - prior to amendment	Feb. 29, 2012 2012 Unsecured Bank Term Loan	Jun. 30, 2011 2012 Unsecured Bank Term Loan	Jun. 30, 2012 2016 Unsecured Senior Bank Term Loan	Jun. 30, 2012 2017 Unsecured Senior Bank Term Loan
Unsecured line of credit and unsecured bank term loan
Unsecured credit facility maximum permitted borrowings												$ 1,500,000,000
Accordion option for Unsecured Credit Facility						500,000,000
Number of extensions to maturity date											2
Length of maturity date extension period											6 months
Base rate												LIBOR				LIBOR	LIBOR
Borrowings	1,350,000,000		1,350,000,000		1,600,000,000											750,000,000	600,000,000
Applicable margin (as a percent)							1.65%	1.20%				1.20%	2.40%			1.65%	1.50%
Annual facility fee (as a percent)												0.25%
Loss on early extinguishment of debt	$ (1,602,000)	$ (1,248,000)	$ (2,225,000)	$ (3,743,000)							$ 1,600,000			$ 600,000	$ 1,200,000
Financial covenants, Requirements
Leverage Ratio (as a percent)									60.00%
Fixed Charge Coverage Ratio										150.00%
Secured Debt Ratio (as a percent)									40.00%
Leverage ratio (as a percent)									60.00%
Interest Coverage Ratio										1.75