Gain on Sales of Investment Properties (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Gain on Sales of Investment Properties [Abstract]
Investment property sales (by property)	12	9	21	19
Proceeds from sales of investment properties	$ 7,300,000	$ 8,900,000	$ 11,700,000
Gain (loss) on sales of investment properties	3,100,000	1,900,000	4,300,000
Excess land sales (in number of properties)	2		5	1
Proceeds from sales of excess land	108,000		783,000
Gain (loss) on sales of excess land	55,000		210,000	0
Proceeds from sales of investment properties and excess land				16,700,000
Gain (loss) on sales of investment properties and excess land				$ 4,300,000