Beneficial Interest - Additional Information (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Beneficial Interest
Common shares, outstanding	9,436,000	8,967,000
Preferred shares, outstanding	0	0
Dividends paid	$ 1.0450	$ 1.0175	$ 0.9900
Total Beneficial Interest
Beneficial Interest
Common shares, authorized	100,000,000
Common shares, par value	$ 0.01
Preferred shares, authorized	50,000,000
Preferred shares, par value	$ 0.01