Senior Unsecured Notes and Secured Debt (Summary of Secured Debt Principal Activity) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Amount
Debt issued	$ 62,055	$ 343,696	$ 45,447
Ending balance	13,887,606
Secured Debt
Amount
Beginning balance	2,993,342	2,485,711	2,618,408
Debt issued	62,055	343,696	45,447
Debt assumed	0	385,145	292,887
Debt extinguished	(632,288)	(230,108)	(306,553)
Principal payments	(62,707)	(54,325)	(56,288)
Foreign currency	17,671	63,223	(108,190)
Ending balance	$ 2,378,073	$ 2,993,342	$ 2,485,711
Weighted Avg. Interest Rate
Beginning balance	3.63%	3.90%	3.76%
Debt issued	0.0255	0.0311	0.0340
Debt assumed	0	0.0434	0.0464
Debt extinguished	0.0221	0.0435	0.0536
Principal payments	0.0363	0.0375	0.0391
Foreign currency	0.0293	0.0328	0.0333
Ending balance	3.27%	3.63%	3.90%