Real Estate Investments - Acquisitions and dispositions (Details) $ in Thousands					3 Months Ended
	Apr. 02, 2019 USD ($) ft²	Mar. 28, 2019 USD ($) ft²	May 21, 2018 USD ($)	May 03, 2018 USD ($) property	Mar. 31, 2019 USD ($) ft²	Apr. 09, 2019 USD ($) ft² Building
Business Acquisition [Line Items]
Financing lease liabilities					$ 14,294
Real Estate Acquisition
Business Acquisition [Line Items]
Purchase price					93,000
Cash consideration					90,700
Real Estate					93,900
Other					$ (3,200)
Square footage | ft²					301,837
Medical Office Building | Washington, D.C. (4) | Real Estate Acquisition
Business Acquisition [Line Items]
Number of properties acquired | property				2
Date Acquired				Mar. 28, 2019
Purchase price				$ 46,000
Cash consideration		$ 45,900
Real Estate		50,200
Other		$ (4,300)
Square footage | ft²		158,338
Financing lease liabilities					$ 14,400
Above-market lease intangibles					5,200
Medical Office Building | Indianapolis, IN (5) | Real Estate Acquisition
Business Acquisition [Line Items]
Date Acquired			Mar. 28, 2019
Purchase price			$ 47,000
Cash consideration		$ 44,800
Real Estate		43,700
Other		$ 1,100
Square footage | ft²		143,499
Prepaid ground leases		$ 800
Below-market lease intangible		$ 900
Medical Office Building | Tucson, Arizona | Real Estate Dispositions
Business Acquisition [Line Items]
Net Real Estate Investment					$ 6,900
Subsequent Event | Medical Office Building | Atlanta, Georgia | Real Estate Acquisition
Business Acquisition [Line Items]
Purchase price	$ 28,000
Square footage | ft²	47,963
Subsequent Event | Medical Office Building | Tucson, Arizona | Real Estate Dispositions
Business Acquisition [Line Items]
Square footage | ft²						67,345
Number of Off-campus Buildings | Building						3
Number of On-campus Buildings | Building						1
Sales price						$ 13,000