Credit Facilities - Additional Information (Detail) (USD $)				12 Months Ended						1 Months Ended				1 Months Ended	2 Months Ended
	Dec. 31, 2011 Entity	Dec. 31, 2010 Entity	Dec. 31, 2009 Entity	Dec. 31, 2011 Revolving Credit Agreement	Dec. 31, 2010 Revolving Credit Agreement	Dec. 31, 2009 Revolving Credit Agreement	Dec. 31, 2011 Revolving Credit Agreement Amendment	Dec. 31, 2011 Revolving Credit Agreement LIBOR	Dec. 31, 2011 Unsecured Revolving Credit Facility	Jul. 05, 2011 Senior Unsecured Notes Issued July Twenty Eleven	Jul. 27, 2005 Senior Unsecured Notes Issued July Twenty Zero Five	Dec. 31, 2011 Senior Unsecured Notes Issued July Twenty Zero Five	Dec. 31, 2010 Senior Unsecured Notes Issued July Twenty Zero Five	Dec. 31, 2002 Related Party Property	Nov. 30, 2004 Related Party Property
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 60,000,000			$ 150,000,000		$ 140,000,000
Interest rate								0.875%
Line of credit amount outstanding				0	0
Commitment fee for unused balance amount				0.20%
Weighted average interest rate on borrowings				0.97%	0.97%	1.23%
Revolving credit agreement, expiry date				2013-05-23
Retained earnings available for dividend payments and stock repurchases				140,100,000
Notes issued										125,000,000	60,000,000
Debt interest rate										3.75%	5.03%
Debt instrument, maturity date										Apr. 27, 2018	Jul. 27, 2012
Debt instrument, interest terms											Interest-only payments were due quarterly for the first two years, followed by annual $12.0 million principal repayments plus interest for the five years thereafter.
Payment of Interest, commencement date										Jul. 27, 2011
Interest only payments period											2 years
Principal plus interest repayment period											5 years
Annual principal repayment amount										25,000,000	12,000,000
Debt Outstanding												12,000,000	24,000,000
Debt principal amount, commencement date										2014-04-27
Properties sold	25	11	37											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															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	$ 3,303,000
Weighted-average borrowing rate on bonds	0.38%