Senior Unsecured Notes and Secured Debt (Details 2) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Secured debt principal activity
Secured debt issued	$ 75,136	$ 82,724
Secured debt extinguished	130,343	208,057
Excluding Fair Value Adjustments [Member]
Secured debt principal activity
Secured debt principal balance	3,488,488	2,972,657	$ 3,478,207	$ 2,941,765
Secured debt issued	75,136	82,724
Secured debt assumed	0	205,897
Secured debt extinguished	(111,701)	(192,427)
Secured debt principal payments	(18,642)	(49,672)
Secured debt foreign currency	$ 65,488	$ (15,630)
Secured debt principal balance average rate	0.044	0.0488	0.0444	0.0494
Secured debt principal issued average rate	0.0306	0.0234
Secured debt principal assumed average rate	0	0.0398
Secured debt principal extinguished average rate	0.0445	0.0402
Secured debt principal payment average rate	0.0454	0.0396
Secured debt principal foreign currency average rate	0.0367	0.0501