Senior Unsecured Notes and Secured Debt (Summary of Senior Unsecured Note Principal Activity) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Unsecured note principal activity
Debt issued	$ 2,824,176	$ 7,500	$ 693,560
Long-term Debt, Amount, ending balance	12,185,695
Senior Unsecured Notes
Unsecured note principal activity
Long-term Debt, Amount, beginning balance	8,417,447	8,260,038	8,645,758
Debt issued	2,850,000	7,500	705,000
Debt extinguished	(1,450,000)	(5,000)	(850,000)
Foreign currency	(117,463)	154,909	(240,720)
Long-term Debt, Amount, ending balance	$ 9,699,984	$ 8,417,447	$ 8,260,038
Weighted average interest rate
Weighted Avg. Interest Rate, beginning balance	4.31%	4.25%	4.24%
Debt issued, weighted avg. interest rate	0.0457	0.0197	0.0423
Debt extinguished, weighted avg. interest rate	0.0346	0.0183	0.0419
Foreign currency, weighted avg. interest rate	0.0416	0.0429	0.0457
Weighted Avg. Interest Rate, ending balance	4.48%	4.31%	4.25%