Secured and unsecured senior debt (Details 4) (USD $)	1 Months Ended	3 Months Ended	9 Months Ended								1 Months Ended	9 Months Ended		1 Months Ended
	Apr. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Apr. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans	Sep. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Less than or equal to	Sep. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Greater than or equal to	Sep. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans Requirement Less than or equal to	Sep. 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 item	Sep. 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
Unsecured line of credit and unsecured bank term loan
Unsecured credit facility maximum permitted borrowings												$ 1,100,000,000
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
Borrowings			1,350,000,000		1,600,000,000
Applicable margin (as a percent)							1.75%	1.20%				1.20%	2.40%
Annual facility fee (as a percent)												0.25%
Loss on early extinguishment of debt		(2,742,000)	(2,225,000)	(6,485,000)							1,600,000			600,000	1,200,000
Principal amount of unsecured convertible notes repurchased	$ 1,000,000													$ 250,000,000	$ 500,000,000
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