Notes Payable, Unsecured Notes and Credit Facility (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Notes Payable, Unsecured Notes and Credit Facility
Total notes payable and unsecured notes	$ 6,029,998	$ 3,854,068
Total mortgage notes payable, unsecured notes and Credit Facility	6,029,998	3,854,068
Unsecured notes
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	2,600,000	1,950,000
Total notes payable and unsecured notes	2,600,000
Amount of debt discount	5,291	4,202
Mortgage notes payable
Notes Payable, Unsecured Notes and Credit Facility
Fixed rate notes	2,418,389	1,427,133
Variable rate notes	1,011,609	476,935
Total notes payable and unsecured notes	3,222,763
Amount of debt premium	$ 120,684	$ 1,167