Senior Unsecured Notes and Secured Debt (Details 2) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Secured debt principal activity
Net proceeds from the issuance of secured debt	$ 190,459	$ 193,541
Secured debt extinguished	1,050,879	471,898
Excluding Fair Value Adjustments [Member]
Secured debt principal activity
Secured debt principal balance	2,720,002	3,300,642	$ 3,465,066	$ 3,478,207
Net proceeds from the issuance of secured debt	190,459	193,541
Secured debt assumed	23,094	47,156
Secured debt extinguished	(1,003,372)	(416,009)
Secured debt principal payments	(47,507)	53,636
Secured debt foreign currency	$ 92,262	$ (55,889)
Secured debt principal balance average rate	0.03735	0.04272	0.04094	0.0444
Secured debt principal issued average rate	0.0273	0.03053
Secured debt principal assumed average rate	0.0667	0.04132
Secured debt principal extinguished average rate	0.05321	0.04986
Secured debt principal payment average rate	0.04339	0.03631
Secured debt principal foreign currency average rate	0.03196	0.04478