Fair value measurements Book and Fair Values (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
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	$ 112,547	$ 137,621
Interest rate hedge assets	110	596
Secured notes payable	722,794	809,818
Unsecured senior notes payable	2,376,713	2,030,631
Long-term Line of Credit	72,000	151,000
Unsecured senior bank term loans	945,030	944,243
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	112,547	137,621
Interest rate hedge assets	110	596
Interest Rate Derivative Liabilities, at Fair Value	12,332	4,314
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	112,547	137,621
Interest rate hedge assets	110	596
Interest Rate Derivative Liabilities, at Fair Value	12,332	4,314
Secured notes payable	741,424	832,342
Senior Notes Fair Value Disclosure	2,513,945	2,059,855
Unsecured senior line of credit	72,098	151,450
Unsecured Bank Term Loans Fair Value Disclosure	$ 956,916	$ 951,098