Senior Unsecured Notes and Secured Debt (Details 1) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Unsecured note issuances
Senior unsecured debt redeemed	$ 450,000	$ 0
Excluding Fair Value Adjustments [Member]
Unsecured note issuances
Senior unsecured debt balance	8,006,989	8,284,267	$ 8,417,447	$ 8,260,038
Senior unsecured debt extinguished	(450,000)	0
Senior unsecured debt foreign currency	$ 39,542	$ 24,229
Senior unsecured debt balance average rate	0.04451	0.04262	0.04306	0.04245
Senior unsecured debt extinguished average rate	0.0225	0
Senior unsecured debt foreign currency average rate	0.05216	0.04391