Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	$ 3,435	$ 2,709
Book Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	3,435	2,709
Interest rate swap agreements	(12,547)	(20,661)
Secured notes payable	(711,029)	(716,144)
Unsecured senior notes payable	(1,048,395)	(549,805)
Unsecured senior line of credit	0	(566,000)
Unsecured senior bank term loans	(1,200,000)	(1,350,000)
Fair Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	3,435	2,709
Interest rate swap agreements	(12,547)	(20,661)
Secured notes payable	(766,262)	(788,455)
Unsecured senior notes payable	(1,039,170)	(593,350)
Unsecured senior line of credit	0	(567,196)
Unsecured senior bank term loans	$ (1,357,376)	$ (1,405,124)