Shareholders' Equity - Additional Information (Details) $ / shares in Units, $ in Thousands											1 Months Ended
	Oct. 12, 2017 USD ($) $ / shares	Sep. 19, 2017 $ / shares shares	Sep. 14, 2017 $ / shares shares	Aug. 21, 2017 USD ($) $ / shares	Aug. 03, 2017 USD ($) $ / shares shares	Jul. 05, 2017 $ / shares shares	Jun. 30, 2017 $ / shares shares	May 22, 2017 USD ($) $ / shares	May 17, 2017 USD ($) trustee $ / shares shares	Feb. 23, 2017 USD ($) $ / shares	Sep. 30, 2017 shares	Sep. 30, 2016 shares
Distributions
Cash distribution to common shareholders (in dollars per share)				$ 0.43				$ 0.43		$ 0.43
Distribution paid to common shareholders | $				$ 41,364				$ 30,606		$ 30,606
Proceeds from offering | $					$ 493,936
Number of trustees | trustee									6
Shares withheld for tax withholding (in shares) | shares									450
Value of shares withheld for tax withholding | $									$ 10
Subsequent Event
Distributions
Distribution paid to common shareholders | $	$ 42,633
Distribution payable to common shareholders (in dollars per share)	$ 0.43
Common shares | Trustees
Distributions
Common shares granted (in shares) | shares									3,000
Value of shares granted (in dollars per share)									$ 21.75
Common shares | Officers and certain employees
Distributions
Common shares granted (in shares) | shares			57,350
Value of shares granted (in dollars per share)			$ 18.61
Underwritten Public Offering
Distributions
Number of shares sold in offering (in shares) | shares						25,000,000
Price per share in offering (in dollars per share)						$ 18.50
Over-Allotment Option
Distributions
Number of shares sold in offering (in shares) | shares					2,907,029
Price per share in offering (in dollars per share)					$ 18.50
Former Employee of RMR LLC
Distributions
Stock repurchased to satisfy tax withholding obligation (in shares) | shares		13,636					278				13,636	13,209
Stock repurchased to satisfy tax withholding obligation (in dollars per share)		$ 18.30					$ 18.31