2017 Acquisitions (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Payments to acquire real estate | $	$ 265,907	$ 664,684	$ 1,413,595
Acquisitions in 2017
Business Acquisition [Line Items]
Payments to acquire real estate | $		$ 691,300
Triple Net Leased Properties
Business Acquisition [Line Items]
Number of real estate properties acquired		15
Properties acquired previously owned by equity method investee		6
Office Operations
Business Acquisition [Line Items]
Number of real estate properties acquired	5	4
Office Operations | Research and Innovation Center
Business Acquisition [Line Items]
Number of real estate properties acquired	1	3
Office Operations | Medical Office Buildings
Business Acquisition [Line Items]
Number of real estate properties acquired	4	1
Seniors Housing Communities
Business Acquisition [Line Items]
Number of real estate properties acquired	1	3