Real Estate Properties - Acquisition Activities and Pro Forma Financial Information (Details) $ / shares in Units, $ in Thousands, ft² in Millions					1 Months Ended	9 Months Ended
	Jul. 01, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) ft² property	Dec. 27, 2018 shares	Oct. 09, 2018 USD ($) $ / shares shares	Jul. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Business Acquisition [Line Items]
Consideration transferred					$ 2,900
Proceeds from sale of RMR Inc. common shares, net						$ 104,674	$ 0
ILPT | SIR
Business Acquisition [Line Items]
Shares holding (in shares) | shares			45,000,000
Underwritten public offering
Business Acquisition [Line Items]
Price per share (in dollars per share) | $ / shares	$ 40.00
Proceeds from sale of RMR Inc. common shares, net	$ 104,674
Underwritten public offering | Common shares
Business Acquisition [Line Items]
Shares sold (in shares) | shares				24,918,421
Price per share (in dollars per share) | $ / shares				$ 18.25
Proceeds from sale of RMR Inc. common shares, net				$ 435,125
SIR
Business Acquisition [Line Items]
Consideration transferred		$ 2,415,053
Number of properties acquired | property		99
Rentable area of properties (in square feet) | ft²		16.5
Closing costs		$ 27,497
Payments for closing costs		14,508
Debt assumed		1,719,772
SIR | SIR
Business Acquisition [Line Items]
Payments for closing costs		$ 12,989