Mortgage Notes Payable (Narrative) (Details) (USD $)	3 Months Ended		0 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 12, 2012 Bay Colony Corporate Center [Member]	Mar. 31, 2012 Bay Colony Corporate Center [Member]	Mar. 31, 2012 Montvale Center [Member]
Mortgage Notes Payable [Line Items]
Debt default, accrued interest rate					9.93%
Amount of debt extinguished					$ 25,000,000
Interest at a fixed rate			6.53%
Maturity date				Jun. 11, 2012	Jun. 06, 2012
Discontinued operations gain	17,807,000				17,807,000
Repayments of mortgage notes payable	150,519,000	8,777,000	143,900,000
Penalty for early repayment of debt			0
Gains from early extinguishments of debt	$ 767,000		$ 900,000