Beneficial Interest - Additional Information (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Beneficial Interest
Common shares, outstanding	8,001,000	7,579,000
Preferred shares, outstanding	0	0
Dividends paid	$ 0.9600	$ 0.9300	$ 0.9000
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