Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment									2
Rental income	$ 107,170	$ 70,179	$ 69,887	$ 69,296	$ 66,030	$ 64,478	$ 64,061	$ 63,611	$ 316,532	$ 258,180	$ 248,549
Expenses:
Real estate taxes									37,942	30,703	29,906
Utility expenses									20,998	17,269	17,916
Other operating expenses									65,349	54,290	50,425
Depreciation and amortization									109,588	73,153	68,696
Loss on impairment of real estate									9,490	0	0
Acquisition related costs									0	1,191	811
General and administrative									18,847	14,897	14,826
Total expenses									262,214	191,503	182,580
Operating income (loss)									54,318	66,677	65,969
Dividend income									1,216	971	811
Interest income									1,962	158	14
Interest expense									(65,406)	(45,060)	(37,008)
Gain (loss) on early extinguishment of debt									(1,715)	104	34
Loss on distribution to common shareholders of The RMR Group Inc. common stock									0	0	(12,368)
Net gain (loss) on issuance of shares by Select Income REIT									72	86	(42,145)
Loss on impairment of Select Income REIT investment									0	0	(203,297)
Income (loss) from continuing operations before income taxes, equity in earnings of investees and gain on sale of real estate									(9,553)	22,936	(227,990)
Income tax expense									(101)	(101)	(86)
Equity in earnings of investees									21,571	35,518	18,640
Income (loss) from continuing operations									11,917	58,353	(209,436)
Income (loss) from discontinued operations									173	(589)	(525)
Income (loss) before gain on sale of real estate									12,090	57,764	(209,961)
Gain on sale of real estate									0	79	0
Net income (loss)									12,090	57,843	(209,961)
Preferred units of limited partnership distributions									(275)	0	0
Net income (loss) available for common shareholders	(18,266)	$ 10,989	$ 11,677	$ 7,415	12,065	$ 11,578	$ 16,813	$ 17,387	11,815	57,843	(209,961)
Total assets	3,703,565				2,385,066				3,703,565	2,385,066	2,168,510
Investment in SIR
Expenses:
Income (loss) from continuing operations before income taxes, equity in earnings of investees and gain on sale of real estate										86
Operating Segments | Investment in Real Estate
Segment Reporting Information [Line Items]
Rental income									316,532	258,180	248,549
Expenses:
Real estate taxes									37,942	30,703	29,906
Utility expenses									20,998	17,269	17,916
Other operating expenses									65,349	54,290	50,425
Depreciation and amortization									109,588	73,153	68,696
Loss on impairment of real estate									9,490
Acquisition related costs										1,191	561
Total expenses									243,367	176,606	167,504
Operating income (loss)									73,165	81,574	81,045
Interest income									196	124	10
Interest expense									(3,209)	(2,375)	(7,908)
Gain (loss) on early extinguishment of debt										104	34
Income (loss) from continuing operations before income taxes, equity in earnings of investees and gain on sale of real estate									70,152	79,427	73,181
Equity in earnings of investees									(621)
Income (loss) from continuing operations									69,531	79,427	73,181
Income (loss) from discontinued operations									173	(589)	(525)
Income (loss) before gain on sale of real estate										78,838
Gain on sale of real estate										79
Net income (loss)									69,704	78,917	72,656
Net income (loss) available for common shareholders									69,704	78,917	72,656
Total assets	3,138,764				1,807,560				3,138,764	1,807,560	1,639,462
Operating Segments | Investment in SIR
Expenses:
Net gain (loss) on issuance of shares by Select Income REIT									72	86	(42,145)
Loss on impairment of Select Income REIT investment											(203,297)
Income (loss) from continuing operations before income taxes, equity in earnings of investees and gain on sale of real estate									72		(245,442)
Equity in earnings of investees									21,584	35,381	18,620
Income (loss) from continuing operations									21,656	35,467	(226,822)
Income (loss) before gain on sale of real estate										35,467
Net income (loss)									21,656	35,467	(226,822)
Net income (loss) available for common shareholders									21,656	35,467	(226,822)
Total assets	467,499				487,708				467,499	487,708	491,369
Corporate
Expenses:
Acquisition related costs											250
General and administrative									18,847	14,897	14,826
Total expenses									18,847	14,897	15,076
Operating income (loss)									(18,847)	(14,897)	(15,076)
Dividend income									1,216	971	811
Interest income									1,766	34	4
Interest expense									(62,197)	(42,685)	(29,100)
Gain (loss) on early extinguishment of debt									(1,715)
Loss on distribution to common shareholders of The RMR Group Inc. common stock											(12,368)
Income (loss) from continuing operations before income taxes, equity in earnings of investees and gain on sale of real estate									(79,777)	(56,577)	(55,729)
Income tax expense									(101)	(101)	(86)
Equity in earnings of investees									608	137	20
Income (loss) from continuing operations									(79,270)	(56,541)	(55,795)
Income (loss) before gain on sale of real estate										(56,541)
Net income (loss)									(79,270)	(56,541)	(55,795)
Preferred units of limited partnership distributions									(275)
Net income (loss) available for common shareholders									(79,545)	(56,541)	(55,795)
Total assets	$ 97,302				$ 89,798				$ 97,302	$ 89,798	$ 37,679