Credit Facilities - Additional Information (Detail) (USD $)	12 Months Ended			12 Months Ended				1 Months Ended		1 Months Ended	2 Months Ended				12 Months Ended
	Dec. 31, 2012 Entity	Dec. 31, 2011 Entity	Dec. 31, 2010 Entity	Dec. 31, 2012 Minimum	Dec. 31, 2012 Maximum	Dec. 31, 2012 Amended Credit Facility	Dec. 31, 2012 Revolving Credit Agreement	Jul. 05, 2011 Senior Unsecured Notes Issued July Twenty Eleven	Jul. 31, 2012 Senior Unsecured Notes Issued July Twenty Zero Five	Dec. 31, 2002 Related Party Property	Nov. 30, 2004 Related Party Property	Dec. 31, 2012 Unsecured Revolving Credit Facility	Dec. 31, 2012 Revolving Credit Agreement	Dec. 31, 2011 Revolving Credit Agreement	Dec. 31, 2012 Revolving Credit Agreement Minimum	Dec. 31, 2012 Revolving Credit Agreement Maximum	Dec. 31, 2012 Revolving Credit Agreement LIBOR Minimum	Dec. 31, 2012 Revolving Credit Agreement LIBOR Maximum
Debt Instrument [Line Items]
Revolving credit agreement, expiry date	May 23, 2013					Dec. 13, 2017
Line of Credit Facility, Maximum Borrowing Capacity												$ 140,000,000
Interest rate basis spread																	1.00%	1.50%
Line of credit amount outstanding													0	0
Commitment fee for unused balance amount															0.15%	0.30%
Percentage of cash dividend that can be paid to consolidated net earning							50.00%
Retained earnings available for dividend payments and stock repurchases							137,600,000
Notes issued								125,000,000
Debt interest rate								3.75%
Debt instrument, maturity date								Apr. 27, 2018
Payment of Interest, commencement date								Jul. 27, 2011
Annual principal repayment amount								25,000,000	12,000,000
Debt principal amount, commencement date								Apr. 27, 2014
Properties sold	3	25	11							10	11
Borrowings collateralized by the land and buildings										5,000,000	6,800,000
Lease term										15 years	15 years
Lease renewal option term				1 year	20 years						5 years
Aggregate annual rental										556,000	716,000
Interest implicit in the leases										11.10%	9.70%
Industrial development corporation revenue bonds	$ 3,250,000	$ 3,250,000
Weighted-average borrowing rate on bonds	0.35%	0.29%