Capital Stock (Detail Textuals)	1 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2015 USD ($) properties	May. 31, 2015 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) properties	Jun. 30, 2015 USD ($) properties shares	Jun. 30, 2014 USD ($)
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock				$ 806,500,000	$ 0
Common Class A [Member]
Class of Stock [Line Items]
Issuance of stock in a public offering | shares				37,375,000
May 2015 Common Stock Issuance [Member]
Class of Stock [Line Items]
Net proceeds to retire nonrecourse mortgage debt and revolving credit facility borrowings			$ 295,979,000
Remaining net proceeds from stock issuance expected to reduce debt and for general operating purposes			$ 124,365,000
May 2015 Common Stock Issuance [Member] | Common Class A [Member]
Class of Stock [Line Items]
Issuance of stock in a public offering | shares		37,375,000
Shares Issued, Price Per Share | $ / shares		$ 22.50
Proceeds from Issuance of Common Stock		$ 806,500,000
University Park at MIT [Member]
Class of Stock [Line Items]
Equity ownership percentage acquired	49.00%		49.00%	49.00%
Number of life science office properties | properties	7		7	7
Number of parking facilities	2		2	2
Company's ownership interest after acquisition	100.00%		100.00%	100.00%
University Park at MIT [Member] | May 2015 Common Stock Issuance [Member]
Class of Stock [Line Items]
Net proceeds to finance acquisition	$ 386,156,000			$ 386,156,000