Fair value measurements Book and Fair Values (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Book Value
Summary of investments in privately held entities that report NAV, secured notes payable, unsecured senior line of credit, and unsecured term loans
Secured notes payable	$ 775,689	$ 771,061
Unsecured senior notes payable	3,396,912	3,395,804
Unsecured senior line of credit	490,000	50,000
Unsecured senior bank term loans	548,197	547,942
Fair Value
Summary of investments in privately held entities that report NAV, secured notes payable, unsecured senior line of credit, and unsecured term loans
Secured notes payable	774,627	776,222
Unsecured senior notes payable	3,426,630	3,529,713
Unsecured senior line of credit	489,133	49,986
Unsecured senior bank term loans	545,356	549,361
Investment in privately held entities that report NAV | Book Value
Summary of investments in privately held entities that report NAV, secured notes payable, unsecured senior line of credit, and unsecured term loans
Investments	265,466	0
Investment in privately held entities that report NAV | Fair Value
Summary of investments in privately held entities that report NAV, secured notes payable, unsecured senior line of credit, and unsecured term loans
Investments	$ 265,466	$ 0