Incentive Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity Incentive Plan [Line Items]
Number of options outstanding	0	0	23,000	36,000
Dividend equivalent rights, outstanding	0	0	23,000
Dividend equivalent rights paid to officers and Trustees	$ 0	$ 30,000	$ 75,000
Number of stock options exercised during the period, shares	0	23,000	13,000
Value of stock options exercised during the period		$ 520,420	$ 154,500
Incentive Plan 2007
Equity Incentive Plan [Line Items]
Number of shares authorized for issuance	125,000
Shares issued, net of cancellation	85,765
Number of shares vested	67,555
Number of shares remaining for issuance	39,235
Restricted shares issued, net of cancellation	9,105	9,105	9,730
Restricted shares issued, weighted average grant price	$ 73.46	$ 56.34	$ 47.75
Restricted shares of beneficial interest, net of cancellations, issued, value	$ 668,853	$ 512,976	$ 464,608
Restricted shares, Vesting Period	2019-06	2018-06	2017-06
Compensation expenses	$ 538,000	$ 481,000	$ 427,000
Compensation expenses remained to be recognized in future	$ 581,000
Compensation expenses not yet recognized, period of recognition	1 year
1997 Plan
Equity Incentive Plan [Line Items]
Compensation expenses	$ 0
Options granted, vesting percentage per year starting from one year from date of grant	25.00%
Options, expiration period	10 years
Number of options outstanding	0	0