Senior Unsecured Notes and Secured Debt (Details 2) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Secured debt principal activity
Secured debt issued	$ 20,326	$ 12,536
Secured debt extinguished	197,655	822,438
Excluding Fair Value Adjustments [Member]
Secured debt principal activity
Secured debt principal balance	2,498,395	2,665,477	$ 2,618,408	$ 3,465,066
Secured debt issued	20,326	12,536
Secured debt assumed	85,192	0
Secured debt extinguished	(183,408)	(806,189)
Secured debt foreign currency	(27,876)	10,313
Secured debt principal payments	$ (14,247)	$ (16,249)
Secured debt principal balance average rate	0.03696	0.03744	0.03761	0.04094
Secured debt principal issued average rate	0.03774	0.0234
Secured debt principal assumed average rate	0.04395	0
Secured debt principal extinguished average rate	0.05809	0.0558
Secured debt principal foreign currency average rate	0.03326	0.03262
Secured debt principal payment average rate	0.03865	0.04469