Senior Unsecured Notes and Secured Debt (Details 1) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unsecured note principal activity
Senior unsecured debt issued	$ 1,756,192,000	$ 2,025,708,000	$ 1,381,086,000
Senior unsecured debt redeemed	517,625,000	370,524,000	3,000
Unsecured Debt [Member]
Unsecured note principal activity
Senior unsecured debt balance	7,186,678,000	5,894,403,000	4,464,927,000	3,064,930,000
Senior unsecured debt issued	1,786,930,000	1,800,000,000	1,400,000,000
Senior unsecured debt extinguished	(300,000,000)	(76,853,000)	(3,000)
Senior unsecured debt redeemed	(219,295,000)	(293,671,000)	0
Senior unsecured debt foreign currency	$ 24,640,000	$ 0	$ 0
Senior unsecured debt balance average rate	0.04456	0.04675	0.05133	0.05129
Senior unsecured debt issued average rate	0.03824	0.03691	0.05143
Senior unsecured debt extinguished average rate	0.06	0.08	0.0475
Senior unsecured debt redeemed average rate	0.03	0.0475	0
Senior unsecured debt foreign currency average rate	0.048	0	0