Real Estate Properties - Purchase Price Allocation for SIR (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Purchase Price Allocation:
Assumed working capital		$ 0	$ 25,170	$ (1,596)
Select Income REIT unsecured revolving credit facility repaid at closing	$ 7,890
SIR
Purchase Price Allocation:
Land			477,977
Buildings and improvements			956,801
Assets of properties held for sale			6,846
Acquired real estate leases			854,431
Cash			24,744
Restricted cash			476
Rents receivable			11,370
Other assets			88,658
Total assets			2,421,303
Accounts payable and other liabilities			61,289
Assumed real estate lease obligations			11,879
Due to related persons			30,120
Net assets acquired			639,578
Assumed working capital			50,390
Select Income REIT unsecured revolving credit facility repaid at closing			108,000
Consideration transferred and liabilities assumed			2,409,740
Line of Credit | SIR
Purchase Price Allocation:
Long term debt acquired			108,000
Senior Unsecured Notes | SIR
Purchase Price Allocation:
Long term debt acquired			1,410,947
Assumed mortgage notes			1,450,000
Principal balance			1,450,000
Mortgage Note Payable | SIR
Purchase Price Allocation:
Long term debt acquired			159,490
Assumed mortgage notes			161,772
Principal balance			161,772
RMR LLC | SIR
Purchase Price Allocation:
Investment at fair value			$ 84,229