Senior Unsecured Notes and Secured Debt (Details 1) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Unsecured note issuances
Senior unsecured debt issued	$ 497,862,000	$ 842,323,000
Senior unsecured debt redeemed	217,615,000	370,524,000
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	6,175,118,000	4,694,403,000	5,894,403,000	4,464,927,000
Senior unsecured debt issued	500,000,000	600,000,000
Senior unsecured debt extinguished	0	(76,853,000)
Senior unsecured debt redeemed	$ (219,285,000)	$ (293,671,000)
Senior unsecured debt balance average rate	0.0448	0.0503	0.04675	0.05133
Senior unsecured debt issued average rate	0.01552	0.04125
Senior unsecured debt extinguished average rate	0	0.08
Senior unsecured debt redeemed average rate	0.03	0.0475