Gain on Sales of Investment Properties (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013 item	Sep. 30, 2012 item	Sep. 30, 2013 item	Sep. 30, 2012 item
Sales of investment properties
Sales proceeds			$ 98,768,000	$ 34,283,000
New properties and properties under development
Sales of investment properties
Number of properties sold	19	11	53	30
Sales proceeds	22,400,000	15,800,000	106,100,000	34,300,000
Gain on sales of investment properties	$ 6,200,000	$ 2,000,000	$ 50,500,000	$ 6,000,000