Senior Unsecured Notes and Secured Debt (Details 1) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
Unsecured note issuances
Senior unsecured debt issued	$ 842,323	$ 1,381,086
Senior unsecured debt redeemed	370,524	0
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	4,694,403	4,464,930	4,464,927	3,064,930
Senior unsecured debt issued	600,000	1,400,000
Senior unsecured debt extinguished	(76,853)	0
Senior unsecured debt redeemed	$ (293,671)	$ 0
Senior unsecured debt balance average rate	0.05030	0.05133	0.05133	0.05129
Senior unsecured debt issued average rate	0.04125	0.05143
Senior unsecured debt extinguished average rate	0.08000	0.000
Senior unsecured debt redeemed average rate	0.04750	0.000