Mortgage Notes Payable (Narrative) (Detail) (USD $)	3 Months Ended	6 Months Ended					6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Montvale Center [Member]	Jun. 30, 2012 Bay Colony Corporate Center [Member]	Mar. 12, 2012 Bay Colony Corporate Center [Member]	Jun. 30, 2012 One Freedom Square [Member]	Apr. 02, 2012 One Freedom Square [Member]
Mortgage Notes Payable [Line Items]
Debt default, accrued interest rate				9.93%
Amount of debt extinguished				$ 25,000,000
Maturity Date				Jun. 06, 2012	Jun. 11, 2012		Jun. 30, 2012
Gain on forgiveness of debt from discontinued operations		17,807,000		17,807,000
Repayments of mortgage notes payable		219,865,000	468,639,000		143,900,000		65,100,000
Interest at a fixed rate						6.53%		7.75%
Penalty for early repayment of debt					0		0
Gains from early extinguishments of debt	$ 274,000	$ 1,041,000			$ 900,000		$ 300,000