Notes Payable, Unsecured Notes and Credit Facility (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Notes Payable, Unsecured Notes and Credit Facility
Total notes payable and unsecured notes	$ 3,854,068	$ 3,600,025
Total mortgage notes payable, unsecured notes and Credit Facility	3,854,068	3,600,025
Amount of debt discount	4,202	1,802
Debt basis adjustment	0	11
Amount of debt premium	1,167	962
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	1,950,000	1,556,001
Variable rate notes		75,000
Total notes payable and unsecured notes	1,950,000
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	1,427,133	1,528,783
Variable rate notes	476,935	440,241
Total notes payable and unsecured notes	$ 1,728,377