Secured and unsecured senior debt Summary of financial covenants (Details)	Dec. 31, 2013
Less than or equal to | Unsecured Senior Line of Credit and Bank Term Loans
Debt Instrument [Line Items]
Leverage Ratio (as a percent)	60.00%	[1]
Secured Debt Ratio (as a percent)	45.00%	[1]
Leverage Ratio (as a percent)	60.00%	[1]
Greater than or equal to | Unsecured Senior Line of Credit and Bank Term Loans
Debt Instrument [Line Items]
Fixed Charge Coverage Ratio	150.00%	[1]
Interest Coverage Ratio	1.50	[1]
Requirement | Less than or equal to | 4.60% unsecured senior notes payable
Debt Instrument [Line Items]
Total Debt to Total Assets (as a percent)	60.00%	[2]
Secured Debt to Total Assets (as a percent)	40.00%	[2]
Requirement | Greater than or equal to | 4.60% unsecured senior notes payable
Debt Instrument [Line Items]
Fixed Charge Coverage Ratio	150.00%	[2]
Unencumbered Total Asset Value to Unsecured Debt (as a percent)	150.00%	[2]

[1] For a definition of the ratios, refer to the amended unsecured senior line of credit and unsecured senior bank term loan agreements, each dated as of August 30, 2013 and July 26, 2013, which are filed as exhibits to our Quarterly Report on Form 10-Q filed with the SEC on November 7, 2013.

[2] For a definition of the ratios used in the table above, refer to the Indenture dated June 7, 2013, which governs the unsecured senior notes payable, which was filed as an exhibit to the Form 8-K filed with the SEC on June 7, 2013.