Real Estate Activity During Nine Months Ended September 30, 2012 (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended		9 Months Ended						7 Months Ended	9 Months Ended		6 Months Ended	9 Months Ended		1 Months Ended	9 Months Ended	1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Bedford Business Park [Member]	May 17, 2012 Bedford Business Park [Member] sqft	Sep. 30, 2012 Montvale Center [Member]	Jan. 31, 2012 Montvale Center [Member] sqft	Sep. 30, 2012 Series Four Preferred Units [Member]	May 17, 2012 Office/Technical Properties [ Member] Bedford Business Park [Member] Property	May 17, 2012 Class A Office Properties [ Member] Bedford Business Park [Member] Property	Jan. 03, 2012 Two Patriots Park [Member] sqft	Apr. 30, 2012 510 Madison Avenue [Member] sqft	May 04, 2012 One Patriots Park [Member] sqft	Sep. 30, 2012 453 Ravendale Drive [Member]	Sep. 30, 2012 453 Ravendale Drive [Member]	Mar. 01, 2012 453 Ravendale Drive [Member] sqft	Sep. 30, 2012 100 Federal Street [Member] sqft	Sep. 30, 2012 100 Federal Street [Member] Floors sqft	Mar. 13, 2012 100 Federal Street [Member]	Aug. 29, 2012 680 Folsom Street [Member] sqft	Sep. 30, 2012 680 Folsom Street [Member]	Aug. 29, 2012 680 Folsom Street [Member] Series Four Preferred Units [Member]	Aug. 29, 2012 690 Folsom Street [Member] Floors sqft
Real Estate Properties [Line Items]
Acquisition date																Mar. 01, 2012			Mar. 13, 2012		Aug. 29, 2012
Acquisition aggregate purchase price																	$ 6,700,000			$ 615,000,000
Net rentable area						470,000		123,000				256,000	356,000	268,000			30,000	1,264,000	1,264,000		522,000			22,000
Square footage of signed lease																			735,000
Number of floors in building																			37					2
Total revenues	470,904,000	449,703,000	1,389,028,000	1,298,852,000											300,000			36,500,000
Operating income	162,822,000	163,830,000	480,552,000	454,786,000											100,000			6,300,000
Percentage of project pre-leased																					85.00%
Estimated project cost																					340,000,000
Consideration paid																					62,200,000
Series Four Preferred Units																					1,588,100
Preference shares per unit liquidation preference									$ 50.00														$ 50.00
Quarterly distributions									$ 0.25														$ 0.25
Series Four Preferred Units annual dividend payable rate									2.00%														2.00%
Construction loan Facility Borrowing Capacity																					170,000,000
Construction loan facility borrowing capacity amount outstanding																						0
Debt default, accrued interest rate							9.93%
Amount of debt extinguished							25,000,000
Maturity Date							Jun. 06, 2012															May 30, 2015
Gain on forgiveness of debt from discontinued operations			17,807,000				17,807,000
Disposition price					62,800,000
Net cash proceeds from sale of real estate properties			61,963,000		62,000,000
Gain on sale of real estate properties			$ 36,877,000		$ 36,900,000
Number of real estate properties										2	1