Senior Unsecured Notes and Secured Debt (Summary of Senior Unsecured Note Principal Activity) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Amount
Debt issued	$ 1,588,549	$ 3,974,559	$ 2,824,176
Ending balance	13,887,606
Senior Unsecured Notes
Amount
Beginning balance	10,427,562	9,699,984	8,417,447
Debt issued	1,600,000	3,987,790	2,850,000
Debt extinguished	(566,248)	(3,335,290)	(1,450,000)
Foreign currency	48,219	75,078	(117,463)
Ending balance	$ 11,509,533	$ 10,427,562	$ 9,699,984
Weighted Avg. Interest Rate
Beginning balance	4.03%	4.48%	4.31%
Debt issued	0.0189	0.0334	0.0457
Debt extinguished	0.0326	0.0439	0.0346
Foreign currency	0.0435	0.0422	0.0416
Ending balance	3.67%	4.03%	4.48%