Senior Unsecured Notes and Secured Debt (Details 2) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Secured debt principal activity
Secured debt issued	$ 241,772	$ 460,015	$ 228,685
Secured debt extinguished	1,144,346	563,759	573,390
Secured Debt [Member]
Secured debt principal activity
Secured debt principal balance	2,618,408	3,465,066	3,478,207	$ 2,941,765
Secured debt issued	241,772	460,015	228,685
Secured debt assumed	23,094	60,898	1,007,482
Secured debt extinguished	(1,080,268)	(489,293)	(506,326)
Secured debt principal payments	(64,078)	(74,466)	(67,064)
Secured debt deconsolidated	(60,000)	0	0
Secured debt foreign currency	$ 92,822	$ 29,705	$ (126,335)
Secured debt principal balance average rate	0.03761	0.04094	0.0444	0.0494
Secured debt principal issued average rate	0.02822	0.02646	0.02776
Secured debt principal assumed average rate	0.0667	0.04301	0.03334
Secured debt principal extinguished average rate	0.05247	0.05105	0.04506
Secured debt principal payment average rate	0.0434	0.04663	0.04801
Secured debt deconsolidated average rate	0.03799	0	0
Secured debt principal foreign currency average rate	0.03164	0.0367	0.03834