Investments in real estate, net Acquisitions (Details) (USD $)	9 Months Ended		0 Months Ended
	Sep. 30, 2014 sqft	Sep. 30, 2013	Mar. 27, 2014 Greater Boston, Massachusetts 225 Second Avenue sqft	Jan. 30, 2014 San Diego, California 3545 Cray Court sqft	Mar. 17, 2014 San Diego, California 4025/4031/4045 Sorrento Valley Boulevard sqft property	Apr. 18, 2014 San Francisco, California [Member] 510 Townsend Street [Member] sqft	Apr. 18, 2014 San Francisco, California [Member] 1455/1515 Third Street [Member]	Sep. 30, 2014 Secured Debt Maturing on January 2023 San Diego, California 3545 Cray Court	Sep. 30, 2014 Secured Debt Maturing On 15 April 2016	Sep. 30, 2014 Secured Debt Maturing On 15 April 2016 San Diego, California 4025/4031/4045 Sorrento Valley Boulevard
Business Acquisition [Line Items]
Area of Real Estate Property	18,458,379		112,500	116,556	42,566	300,000
Payments to acquire real estate property			$ 16,300,000	$ 64,000,000	$ 12,400,000	$ 50,000,000	$ 125,000,000
Average percentage of occupied properties				100.00%	100.00%
Assumption of secured notes payable in connection with purchase of properties	$ (48,329,000)	$ 0		$ 40,700,000	$ 7,600,000
Number of real estate properties					3
Stated interest rate (as a percent)								4.66%	5.74%	5.74%