Secured and unsecured senior debt (Details 3) (USD $) In Millions, unless otherwise specified	1 Months Ended					1 Months Ended
	Apr. 30, 2012	Feb. 29, 2012 Unsecured senior notes payable, due April 1, 2022	Jun. 30, 2012 Unsecured senior notes payable, due April 1, 2022	Jun. 30, 2012 Unsecured senior notes payable, due April 1, 2022 Requirement Less than or equal to	Jun. 30, 2012 Unsecured senior notes payable, due April 1, 2022 Requirement Greater than or equal to	Feb. 29, 2012 2012 Unsecured Bank Term Loan
Unsecured senior notes payable
Principal amount raised		$ 550
Stated interest rate (as a percent)		4.60%	4.60%
Price as a percentage of principal amount of notes payable		99.915%
Yield rate (as a percent)		4.61%
Prepayment of outstanding principal balance	$ 1.0					$ 250.0
Requirements of the key financial covenants under our unsecured senior notes payable
Total Debt to Total Assets (as a percent)				60.00%
Consolidated EBITDA to interest expense (as a percent)					150.00%
Unencumbered total asset value to unsecured debt (as a percent)					150.00%
Secured debt to total assets (as a percent)				40.00%