Notes Payable, Unsecured Notes and Credit Facility (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Notes Payable, Unsecured Notes and Credit Facility
Total notes payable and unsecured notes	$ 6,275,178	$ 6,029,998
Total mortgage notes payable, unsecured notes and Credit Facility	6,275,178	6,029,998
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	2,600,000	2,600,000
Total notes payable and unsecured notes	2,850,000
Amount of debt discount	5,082	5,291
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	2,414,295	2,418,389
Variable rate notes	1,010,883	1,011,609
Total notes payable and unsecured notes	3,222,174
Amount of debt premium	$ 111,702	$ 120,684