Senior Unsecured Notes and Secured Debt (Details 2) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011
Secured debt principal activity
Secured debt issued	$ 85,140,000	$ 145,713,000
Secured debt extinguished	575,373,000	272,399,000
Excluding Fair Value Adjustments [Member]
Secured debt principal activity
Secured debt principal balance	3,065,923,000	2,300,520,800	2,311,586,000	2,108,373,000
Secured debt issued	85,140,000	145,713,000
Secured debt assumed	1,241,898,000	311,653,000
Secured debt extinguished	(535,367,000)	(237,259,000)
Secured debt principal payments	(40,006,000)	(28,247,000)
Secured debt foreign currency	$ 2,672,000	$ 287,800
Secured debt principal balance average rate	0.0508	0.0542	0.0514	0.0534
Secured debt principal issued average rate	0.0505	0.0413
Secured debt principal assumed average rate	0.0411	0.0571
Secured debt principal extinguished average rate	0.0352	0.0429
Secured debt principal payment average rate	0.0527	0.0535
Secured debt principal foreign currency average rate	0.0404	0.0562