Fair value measurements Book and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Available-for-sale equity securities	$ 100,622	$ 60,685
Interest rate hedge assets	3,733	4,115
Interest rate hedge liabilities	583	3,587
Secured notes payable	1,153,890	1,011,292
Unsecured senior notes payable	2,801,290	2,378,262
Unsecured senior line of credit	314,000	28,000
Unsecured senior bank term loans	547,860	746,471
Book Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Available-for-sale equity securities	100,622	60,685
Interest rate hedge assets	3,733	4,115
Interest rate hedge liabilities	583	3,587
Secured notes payable	1,153,890	1,011,292
Unsecured senior notes payable	2,801,290	2,378,262
Unsecured senior line of credit	314,000	28,000
Unsecured senior bank term loans	547,860	746,471
Fair Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Available-for-sale equity securities	100,622	60,685
Interest rate hedge assets	3,733	4,115
Interest rate hedge liabilities	583	3,587
Secured notes payable	1,156,769	1,016,782
Unsecured senior notes payable	2,943,568	2,431,470
Unsecured senior line of credit	313,993	27,998
Unsecured senior bank term loans	$ 550,371	$ 750,422