Mortgage Notes Payable (Narrative) (Detail) (USD $)	3 Months Ended	9 Months Ended				9 Months Ended			9 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 680 Folsom Street [Member] Y OptionPlan	Aug. 29, 2012 680 Folsom Street [Member]	Sep. 30, 2012 Montvale Center [Member]	Sep. 30, 2012 Bay Colony Corporate Center [Member]	Mar. 12, 2012 Bay Colony Corporate Center [Member]	Sep. 30, 2012 One Freedom Square [Member]	Apr. 02, 2012 One Freedom Square [Member]	Sep. 30, 2012 Sumner Square [Member]	Sep. 04, 2012 Sumner Square [Member]
Mortgage Notes Payable [Line Items]
Debt default, accrued interest rate						9.93%
Amount of debt extinguished						$ 25,000,000
Maturity Date				May 30, 2015		Jun. 06, 2012	Jun. 11, 2012		Jun. 30, 2012		Sep. 01, 2013
Gain on forgiveness of debt from discontinued operations		17,807,000				17,807,000
Repayments of mortgage notes payable		248,304,000	1,196,074,000				143,900,000		65,100,000		23,200,000
Interest at a fixed rate								6.53%		7.75%		7.35%
Penalty for early repayment of debt							0		0		200,000
Gains (losses) on early extinguishments of debt	(5,494,000)	(4,453,000)					900,000		300,000		(300,000)
Construction loan Facility Borrowing Capacity					170,000,000
Debt instrument basis spread on variable rate					3.70%
Number of extension options				2
Mortgage loan extension option (years)				1
No borrowings outstanding				0
Description of variable rate basis				LIBOR index rate at a maximum of 3.00% per annum
Derivative maturity date				May 30, 2015
LIBOR index rate				3.00%
Interest rate derivative, notional amount				$ 170,000,000