Incentive Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity Incentive Plan [Line Items]
Number of options outstanding	0	23,000	36,000	40,000
Dividend equivalent rights, outstanding	0	23,000	36,000
Dividend equivalent rights paid to officers and Trustees	$ 30,000	$ 75,000	$ 94,000
Number of stock options exercised during the period, shares	23,000	13,000	4,000
Value of stock options exercised during the period	$ 520,420	$ 154,500	$ 44,160
Incentive Plan 2007
Equity Incentive Plan [Line Items]
Number of shares authorized for issuance	125,000
Shares issued, net of cancellation	77,285
Number of shares vested	57,825
Number of shares remaining for issuance	47,715
Restricted shares issued, net of cancellation	9,730	9,730	9,850
Restricted shares issued, weighted average grant price	$ 56.34	$ 47.75	$ 43.21
Restricted shares of beneficial interest, net of cancellations, issued, value	$ 548,188	$ 464,608	$ 425,619
Restricted shares, Vesting Period	2018-06	2017-06	2016-06
Compensation expenses	$ 481,000	$ 427,000	$ 399,000
Compensation expenses remained to be recognized in future	$ 485,000
Compensation expenses not yet recognized, period of recognition	1 year
1997 Plan
Equity Incentive Plan [Line Items]
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