Real Estate Activity During The Six Months Ended June 30, 2012 (Narrative) (Detail) (USD $)	3 Months Ended		6 Months Ended										4 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Montvale Center [Member]	Jun. 30, 2012 Bedford Business Park [Member]	May 17, 2012 Bedford Business Park [Member] sqft	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	May 04, 2012 One Patriots Park [Member] sqft	Apr. 30, 2012 510 Madison Avenue [Member] sqft	Jun. 30, 2012 453 Ravendale Drive [Member]	Jun. 30, 2012 453 Ravendale Drive [Member]	Mar. 01, 2012 453 Ravendale Drive [Member] sqft	Jun. 30, 2012 100 Federal Street [Member] sqft	Jun. 30, 2012 100 Federal Street [Member] Floors sqft	Mar. 13, 2012 100 Federal Street [Member]
Acquisition date														Mar. 01, 2012			Mar. 13, 2012
Acquisition aggregate purchase price															$ 6,700,000			$ 615,000,000
Square footage of real estate property							470,000			256,000	268,000	356,000			30,000	1,264,000	1,264,000
Square footage of signed lease																	735,000
Number of floors in building																	37
Total revenues	472,897,000	433,867,000	918,124,000	849,149,000									200,000			19,500,000
Operating income	173,711,000	153,612,000	317,730,000	290,956,000									40,000			3,300,000
Debt default, accrued interest rate					9.93%
Amount of debt extinguished					25,000,000
Maturity Date					Jun. 06, 2012
Discontinued operations Gain			17,807,000		17,807,000
Disposition price						62,800,000
Net cash proceeds from sale of real estate properties						62,000,000
Gain on sale of real estate properties	$ 36,877,000		$ 36,877,000			$ 36,900,000
Number of real estate properties								2	1