Fair value measurements Book and Fair Values (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
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, at fair value	$ 137,621	$ 74,265
Interest rate swap agreements	596	0
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, at fair value	137,621	74,265
Interest rate swap agreements	4,314	909
Secured notes payable	818,217	652,209
Unsecured senior notes payable	2,046,578	1,747,370
Unsecured senior line of credit	151,000	304,000
Unsecured senior bank term loans	950,000	975,000
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, at fair value	137,621	74,265
Interest rate swap agreements	596	0
Interest rate swap agreements	4,314	909
Secured notes payable	832,342	693,338
Unsecured senior notes payable	2,059,855	1,793,255
Unsecured senior line of credit	151,450	304,369
Unsecured senior bank term loans	$ 951,098	$ 976,010