Senior Unsecured Notes and Secured Debt (Details 1)	9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Unsecured note issuances
Senior unsecured debt issued	$ 743,407,000	$ 0
Senior unsecured debt redeemed	534,546,000	47,591,000
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	7,974,540,000	7,342,328,000	$ 7,817,154,000	$ 7,421,707,000
Senior unsecured debt issued	750,000,000	0
Senior unsecured debt extinguished	(300,000,000)
Senior unsecured debt redeemed	(215,965,000)	(47,660,000)
Senior unsecured debt foreign currency	$ (76,649,000)	$ (31,719,000)
Senior unsecured debt balance average rate	0.0421	0.04388	0.04385	0.044
Senior unsecured debt issued average rate	0.04	0
Senior unsecured debt extinguished average rate	0.062	0
Senior unsecured debt redeemed average rate	0.03	0.03
Senior unsecured debt foreign currency average rate	0.08065	0.03892