Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	$ 6,563		$ 6,563		$ 6,235
Secured notes payable	(719,350)		(719,350)		(724,305)
Unsecured senior notes payable	(549,794)		(549,794)		(84,959)
Unsecured senior line of credit	(413,000)		(413,000)		(370,000)
Unsecured senior bank term loans	(1,350,000)		(1,350,000)		(1,600,000)
Total impairment charges	9,799	994	9,799	994
"Significant Other Observable Inputs"
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Number of real estate properties held for sale	4		4
Rentable square feet related to rental properties held for sale	504,130		504,130
Total impairment charges	9,800
Fair value measured on non-recurring basis | "Significant Other Observable Inputs"
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Number of real estate properties held for sale	4		4
Rentable square feet related to rental properties held for sale	504,130		504,130
Total impairment charges	9,800
Book Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	6,563		6,563		6,235
Interest rate swap agreements	(25,336)		(25,336)		(32,980)
Secured notes payable	(719,350)		(719,350)		(724,305)
Unsecured senior notes payable	(549,794)		(549,794)		(84,959)
Unsecured senior line of credit	(413,000)		(413,000)		(370,000)
Unsecured senior bank term loans	(1,350,000)		(1,350,000)		(1,600,000)
Fair Value
Summary of marketable securities, secured notes payable, unsecured senior line of credit, unsecured term loans, and unsecured senior convertible notes
Marketable securities	6,563		6,563		6,235
Interest rate swap agreements	(25,336)		(25,336)		(32,980)
Secured notes payable	(814,637)		(814,637)		(810,128)
Unsecured senior notes payable	(585,016)		(585,016)		(85,221)
Unsecured senior line of credit	(426,757)		(426,757)		(378,783)
Unsecured senior bank term loans	$ (1,362,120)		$ (1,362,120)		$ (1,603,917)