Senior Unsecured Notes and Secured Debt (Details 1) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Unsecured note issuances
Senior unsecured debt issued	$ 693,560	$ 743,407
Senior unsecured debt redeemed	400,000	534,546
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	8,790,942	7,974,540	$ 8,645,758	$ 7,817,154
Senior unsecured debt issued	705,000	750,000
Senior unsecured debt extinguished	(400,000)	(300,000)
Senior unsecured debt redeemed	0	(215,965)
Senior unsecured debt foreign currency	$ (159,816)	$ (76,649)
Senior unsecured debt balance average rate	0.04264	0.0421	0.04237	0.04385
Senior unsecured debt issued average rate	0.04228	0.04
Senior unsecured debt extinguished average rate	0.03625	0.062
Senior unsecured debt redeemed average rate	0	0.03
Senior unsecured debt foreign currency average rate	0.04443	0.08065