Fair value measurements Book and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
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	$ 632,792	$ 771,061
Unsecured senior notes payable	4,290,906	3,395,804
Unsecured senior line of credit	413,000	50,000
Unsecured senior bank term loans	347,306	547,942
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	632,792	771,061
Unsecured senior notes payable	4,290,906	3,395,804
Unsecured senior line of credit	413,000	50,000
Unsecured senior bank term loans	347,306	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	623,887	776,222
Unsecured senior notes payable	4,294,053	3,529,713
Unsecured senior line of credit	411,428	49,986
Unsecured senior bank term loans	346,743	549,361
Investments in privately held entities that report NAV
Summary of investments in privately held entities that report NAV, secured notes payable, unsecured senior line of credit, and unsecured term loans
Investments in privately held entities that report NAV	325,989	$ 148,627
Investments 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 in privately held entities that report NAV	325,989
Investments 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 in privately held entities that report NAV	$ 325,989