Senior Unsecured Notes and Secured Debt (Details 1) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Unsecured note issuances
Senior unsecured debt issued	$ 688,560	$ 0
Senior unsecured debt redeemed	400,000	154,654
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	8,934,093	7,567,921	$ 8,645,758	$ 7,817,154
Senior unsecured debt issued	700,000	0
Senior unsecured debt extinguished	(400,000)	0
Senior unsecured debt redeemed	0	(154,654)
Senior unsecured debt foreign currency	$ (11,665)	$ (94,579)
Senior unsecured debt balance average rate	0.04266	0.04352	0.04237	0.04385
Senior unsecured debt issued average rate	0.0425	0
Senior unsecured debt extinguished average rate	0.03625	0
Senior unsecured debt redeemed average rate	0	0.03
Senior unsecured debt foreign currency average rate	0.03943	0.04333