Gain on Sales of Investment Properties (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Sales of investment properties
Sales proceeds	$ 50,563,000	$ 22,049,000	$ 25,779,000
New properties and properties under development
Sales of investment properties
Number of properties sold	44	26	28
Sales proceeds	50,600,000	22,000,000	27,200,000
Gain on sales of investment properties	9,900,000	5,200,000	8,700,000
Excess real estate
Sales of investment properties
Number of properties sold		5	1
Sales proceeds		2,100,000
Gain on sales of investment properties		$ 540,000