SUBSEQUENT EVENTS (Probable Acquisitions) (Details) (USD $)	Sep. 30, 2013	Oct. 29, 2013 Subsequent Event [Member] 201 Spear Street [Member] sqft Tenants acre	Oct. 30, 2013 Subsequent Event [Member] 500 West Madison [Member] Tenants acre	Oct. 30, 2013 Subsequent Event [Member] 500 West Madison [Member] Office Space [Member] sqft	Oct. 30, 2013 Subsequent Event [Member] 500 West Madison [Member] Retail Space [Member] sqft
Subsequent Event [Line Items]
Net rentable area		246,563		1,393,435	64,289
Area of land		0.9	2.9
Purchase price	$ 390,557,000	$ 121,000,000	$ 425,000,000
Earnest money deposits		10,000,000	10,000,000
Earnest money deposit, amount that may be forfeited		$ 10,000,000	$ 10,000,000
Percent of occupany		82.00%	93.00%
Number of tenants		19	90