Senior Unsecured Notes and Secured Debt (Details 1) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Unsecured note principal activity
Senior unsecured debt issued	$ 773,992,000	$ 1,756,192,000	$ 2,025,708,000
Senior unsecured debt redeemed	365,188,000	517,625,000	370,524,000
Unsecured Debt [Member]
Unsecured note principal activity
Senior unsecured debt balance	7,817,154,000	7,421,707,000	5,894,403,000	4,464,927,000
Senior unsecured debt issued	838,804,000	2,036,930,000	1,800,000,000
Senior unsecured debt extinguished	(298,567,000)	(300,000,000)	(76,853,000)
Senior unsecured debt redeemed	(59,143,000)	(219,295,000)	(293,671,000)
Senior unsecured debt foreign currency	$ (85,647,000)	$ 9,669,000	$ 0
Senior unsecured debt balance average rate	0.04385	0.04395	0.04675	0.05133
Senior unsecured debt issued average rate	0.04572	0.03824	0.03691
Senior unsecured debt extinguished average rate	0.05855	0.06	0.08
Senior unsecured debt redeemed average rate	0.03	0.03	0.0475
Senior unsecured debt foreign currency average rate	0.04222	0.03993	0