Real Estate (Details 3) (USD $)	3 Months Ended							9 Months Ended	12 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended			1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2012 item	Jun. 11, 2012 sqft	Dec. 31, 2011 Flex/office property located in Amesbury, MA	Apr. 19, 2011 Flex/office property located in Amesbury, MA	Dec. 31, 2011 Flex/office property located in Amesbury, MA	Dec. 31, 2012 Flex/office property located in Amesbury, MA	Dec. 31, 2010 Flex/office property located in Amesbury, MA	Dec. 22, 2011 Flex/office property located in Amesbury, MA sqft	Apr. 30, 2012 Warehouse and distribution facility located in Youngstown, OH	Apr. 19, 2011 Warehouse and distribution facility located in Youngstown, OH	Dec. 31, 2011 Warehouse and distribution facility located in Youngstown, OH	Dec. 31, 2012 Warehouse and distribution facility located in Youngstown, OH	Dec. 31, 2010 Warehouse and distribution facility located in Youngstown, OH	Apr. 20, 2012 Warehouse and distribution facility located in Youngstown, OH sqft
Disposal of real estate properties
Net rentable square feet										190,000						78,000						153,708
Carrying value of property sold		$ 770,052,000				$ 472,254,000		$ 472,254,000	$ 770,052,000							$ 4,200,000						$ 3,000,000
Sales price											4,800,000						3,400,000
Net proceeds from sales of discontinued operations								4,507,000	7,221,000		4,500,000						3,200,000
Gain on sale of real estate								329,000	222,000		300,000						200,000
Amount contributed to total revenue by the property sold	$ 10,719,000	$ 27,236,000	$ 21,236,000	$ 19,475,000	$ 17,540,000	$ 16,552,000	$ 14,841,000	$ 42,112,000	$ 85,487,000			$ 0	$ 0	$ 0	$ 400,000			$ 200,000	$ 2,000,000	$ 0	$ 400,000
Number of properties sold									3