Real Estate Properties (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	9 Months Ended				9 Months Ended					0 Months Ended	9 Months Ended		0 Months Ended		1 Months Ended					0 Months Ended		1 Months Ended
	Jan. 31, 2013 property sqft	Sep. 30, 2013 sqft	Nov. 05, 2013 property	Dec. 31, 2012 sqft	Sep. 30, 2012 sqft	Sep. 30, 2013 SIR	Sep. 30, 2013 Suburban property property	Dec. 31, 2012 Suburban property property	Sep. 30, 2013 CBD property	Dec. 31, 2012 CBD property	Nov. 04, 2013 Sale property sqft	Sep. 30, 2013 Sale property item sqft	Feb. 16, 2012 Sale SIR sqft	Jun. 28, 2013 Sale Suburban property property sqft	Jun. 29, 2013 Sale Suburban property property sqft	Aug. 31, 2013 Sale Suburban property property sqft	Jun. 30, 2013 Sale Suburban property property sqft	May 31, 2013 Sale Suburban property property sqft	Apr. 30, 2013 Sale Suburban property property sqft	Jan. 31, 2013 Sale Suburban property property sqft	Nov. 04, 2013 Agreement to sell property sqft	Mar. 31, 2013 Boston, MA Sale CBD	Mar. 31, 2013 Boston, MA Sale CBD	Jun. 30, 2013 Tukwila, WA Sale
Real estate properties information
Property square feet		38,895,000			61,733,000						4,201,777	2,394,680	21,400,000	143,802	30,105	129,452	356,045	57,250	618,000	1,060,026	77,394
Real estate improvements						$ 75,431
Aggregate sale price of properties sold or agreed to be sold, excluding closing costs											111,400	46,712		5,250	1,600	4,100	16,300	4,025	830	10,250	2,050	1,806		2,551
Mortgage financing related to suburban property sold	7,688																			7,688
Interest rate on real estate mortgage receivable (as a percent)	6.00%																			6.00%
Gain (loss) on sale of properties		$ 1,596													$ 317					$ 1,277			$ 1,596	$ 1,765
Number of Properties Sold	3										29	9								3	2
Number of Real Estate Properties Held for Sale			45				72	39	4	1
Number of buildings sold	18										65	27		1	1	3	2	1	1	18	2
Number of buildings held for sale							172	93	5	1
Property square feet, held for sale		12,704,719		6,673,851
Number of land parcels sold												2