Secured and unsecured senior debt (Details 3) (USD $)	3 Months Ended						1 Months Ended	3 Months Ended	1 Months Ended
	Mar. 31, 2012	Apr. 30, 2012 Unsecured Senior Line of Credit and Bank Term Loans	Mar. 31, 2013 Unsecured Senior Line of Credit and Bank Term Loans Less than or equal to	Mar. 31, 2013 Unsecured Senior Line of Credit and Bank Term Loans Greater than or equal to	Mar. 31, 2013 Unsecured Senior Line of Credit and Bank Term Loans Requirement Less than or equal to	Mar. 31, 2013 Unsecured Senior Line of Credit and Bank Term Loans Requirement Greater than or equal to	Apr. 30, 2012 Unsecured Credit Facility line of credit	Mar. 31, 2013 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
Unsecured line of credit and unsecured bank term loan
Amount available for borrowing							$ 1,500,000,000
Accordion option for Unsecured Credit Facility		500,000,000
Length of maturity date extension period							6 months
Base rate								LIBOR	LIBOR
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	623,000						1,600,000			600,000
Principal amount of unsecured convertible notes repurchased										$ 250,000,000
Financial covenants, Requirements
Leverage Ratio (as a percent)					60.00%
Fixed Charge Coverage Ratio						150.00%
Secured Debt Ratio (as a percent)					40.00%
Unsecured Leverage Ratio (as a percent)					60.00%
Interest Coverage Ratio						1.75