Subsequent events Narrative (Details) (USD $)	12 Months Ended			0 Months Ended		0 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jan. 31, 2014 Secured notes payable from insurance co. maturing on 4/1/14 Alexandria Technology Square Subsequent Event	Jan. 30, 2014 Secured notes payable from insurance co. maturing on 4/1/14 Alexandria Technology Square Subsequent Event	Jan. 30, 2014 Three Five Four Five Cray Court Subsequent Event sqft	Jan. 30, 2014 Three Five Four Five Cray Court Secured Debt Maturing on January 2023 Subsequent Event
Subsequent Event [Line Items]
Area of real estate property						116,556
Payments to acquired real estate property						$ 64,000,000
Average percentage of occupied properties						100.00%
Expected Cash Basis Return Yield						7.00%
Expected Return Yield						7.20%
Non-recourse secured note payable assumed in business combination							40,700,000
Stated interest rate (as a percent)							4.66%
Principal amount	3,062,837,000				208,683,000
Contributions by redeemable noncontrolling interests	$ 0	$ 0	$ 9,000	$ 20,900,000