Real Estate Properties - Summary of Consideration Transferred and Liabilities Assumed (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Total Purchase Price (excluding acquisition costs):
Assumed working capital		$ 0	$ 25,170	$ (1,596)
Select Income REIT unsecured revolving credit facility repaid at closing	$ 7,890
SIR
Total Purchase Price (excluding acquisition costs):
Closing price of OPI common shares (in dollars per share)			$ 6.87
Cash consideration for fractional shares			$ 8
Equity issuance costs			(239)
Value of consideration transferred			639,578
Assumed working capital			50,390
Select Income REIT unsecured revolving credit facility repaid at closing			108,000
Non-cash portion of purchase price			1,770,162
Total consideration transferred and liabilities assumed			2,409,740
Senior Unsecured Notes | SIR
Total Purchase Price (excluding acquisition costs):
Assumed notes, principal balance			1,450,000
Mortgage Note Payable | SIR
Total Purchase Price (excluding acquisition costs):
Assumed notes, principal balance			$ 161,772
Common Shares | SIR
Total Purchase Price (excluding acquisition costs):
OPI common shares issued (in shares)			23,282,704
Closing price of OPI common shares (in dollars per share)			$ 27.48
Value of consideration transferred			$ 639,809