Senior Unsecured Notes and Secured Debt (Details 1) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Unsecured note principal activity
Senior unsecured debt issued	$ 693,560	$ 1,451,434	$ 773,992
Senior unsecured debt redeemed	865,863	558,830	365,188
Unsecured Debt [Member]
Unsecured note principal activity
Senior unsecured debt balance	8,260,038	8,645,758	7,817,154	$ 7,421,707
Senior unsecured debt issued	705,000	1,475,540	838,804
Senior unsecured debt assumed	0	24,621	0
Senior unsecured debt extinguished	(850,000)	(300,000)	(298,567)
Senior unsecured debt redeemed	0	(240,249)	(59,143)
Senior unsecured debt foreign currency	$ (240,720)	$ (131,308)	$ (85,647)
Senior unsecured debt balance average rate	0.04245	0.04237	0.04385	0.04395
Senior unsecured debt issued average rate	0.04228	0.03901	0.04572
Senior unsecured debt assumed average rate	0	0.06	0
Senior unsecured debt extinguished average rate	0.04194	0.062	0.05855
Senior unsecured debt redeemed average rate	0	0.03303	0.03
Senior unsecured debt foreign currency average rate	0.04565	0.03966	0.04222