Senior Unsecured Notes and Secured Debt (Details 1) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Unsecured note issuances
Senior unsecured debt issued	$ 7,500	$ 693,560
Senior unsecured debt redeemed	5,000	400,000
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	8,404,393	8,790,942	$ 8,260,038	$ 8,645,758
Senior unsecured debt issued	7,500	705,000
Senior unsecured debt extinguished	(5,000)	(400,000)
Senior unsecured debt foreign currency	$ 141,855	$ (159,816)
Senior unsecured debt balance average rate	0.04293	0.04264	0.04245	0.04237
Senior unsecured debt issued average rate	0.0194	0.04228
Senior unsecured debt extinguished average rate	0.0183	0.03625
Senior unsecured debt foreign currency average rate	0.04241	0.04443