Investments in real estate Acquisition (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) ft² Rate
Real Estate [Line Items]
Long-term Debt	$ 4,116,537
Effective rate (as a percent)	3.52%
One Kendall Square [Member]
Real Estate [Line Items]
Area of Real Estate Property | ft²	644,771
Area of Land | ft²	172,500
Payments to Acquire Businesses, Net of Cash Acquired	$ 725,000
Long-term Debt	$ 203,000
Effective rate (as a percent) | Rate	4.82%