Gain on Sales of Investment Properties (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 item	Sep. 30, 2011 item	Sep. 30, 2012 item	Sep. 30, 2011 item
Sales of investment properties
Sales proceeds			$ 23,000	$ 593,000
Gain on sale of land				210,000
New properties and properties under development
Sales of investment properties
Number of properties sold	11	12	30	21
Sales proceeds	15,800,000	7,300,000	34,300,000	11,900,000
Gain on sales of investment properties	2,000,000	3,100,000	6,000,000	4,300,000
Excess land
Sales of investment properties
Number of properties sold		2		4
Sales proceeds		108,000		593,000
Gain on sale of land		$ 55,000		$ 210,000