Senior Unsecured Notes and Secured Debt (Details 1) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Unsecured note principal activity
Senior unsecured debt issued	$ 1,451,434	$ 773,992	$ 1,756,192
Senior unsecured debt redeemed	558,830	365,188	517,625
Unsecured Debt [Member]
Unsecured note principal activity
Senior unsecured debt balance	8,645,758	7,817,154	7,421,707	$ 5,894,403
Senior unsecured debt issued	1,475,540	838,804	2,036,930
Senior unsecured debt assumed	24,621	0	0
Senior unsecured debt extinguished	(300,000)	(298,567)	(300,000)
Senior unsecured debt redeemed	(240,249)	(59,143)	(219,295)
Senior unsecured debt foreign currency	$ (131,308)	$ (85,647)	$ 9,669
Senior unsecured debt balance average rate	0.04237	0.04385	0.04395	0.04675
Senior unsecured debt issued average rate	0.03901	0.04572	0.03824
Senior unsecured debt assumed average rate	0.06	0	0
Senior unsecured debt extinguished average rate	0.062	0.05855	0.06
Senior unsecured debt redeemed average rate	0.03303	0.03	0.03
Senior unsecured debt foreign currency average rate	0.03966	0.04222	0.03993