Mortgage Notes Payable (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended						3 Months Ended	12 Months Ended						0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 12, 2012 Bay Colony Corporate Center [Member]	Apr. 02, 2012 One Freedom Square [Member]	Aug. 29, 2012 680 Folsom Street [Member] Y	Sep. 04, 2012 Sumner Square [Member]	Oct. 04, 2012 Fountain Square [Member]	Jan. 31, 2012 Montvale Center [Member]	Mar. 31, 2012 Montvale Center [Member]	Dec. 31, 2012 Minimum [Member]	Dec. 31, 2011 Minimum [Member]	Dec. 31, 2012 Fixed Rate Mortgages [Member]	Dec. 31, 2011 Fixed Rate Mortgages [Member]	Dec. 31, 2012 Variable Rate Mortgages [Member]	Dec. 31, 2011 Variable Rate Mortgages [Member]	Oct. 04, 2012 Fountain Square [Member]	Oct. 04, 2012 Fountain Square [Member]
Mortgage Notes Payable [Line Items]
Mortgage notes payable	$ 3,102,485,000	$ 3,123,267,000							$ 25,000,000				$ 3,100,000,000	$ 3,100,000,000
Number Real Estate Assets Used For Mortgage Loan Collateral											1	1
Final maturity date of mortgage notes	Apr. 10, 2022
Contractual interest rate range, inimum													4.75%	4.75%
Contractual interest rate range, maximum													9.93%	9.93%
Contractual interest rate, weighted vverage interest rate													5.65%	5.75%
Number of variable rate mortgage loans															0	0
London Interbank Offered Rate															0.21%	0.30%
Debt default, accrued interest rate									9.93%
Maturity date				Jun. 11, 2012	Jun. 30, 2012	May 30, 2015	Sep. 01, 2013	Oct. 11, 2016	Jun. 06, 2012								Oct. 11, 2016
Companys Joint Venture Ownership Percentage								50.00%									50.00%
mortgage loan number of extensions						2
Mortgage Loan Extension Option Years						1
Gain on forgiveness of debt from discontinued operations										17,800,000
Repayments of mortgage notes payable	253,877,000	1,251,841,000	730,191,000	143,900,000	65,100,000		23,200,000
Interest at a fixed rate				6.53%	7.75%		7.35%	5.71%									5.71%
Penalty for early repayment of debt				0	0		200,000
Noncash or Part Noncash Acquisition, Debt Assumed																	211,300,000	211,300,000
Gains (Losses) on Extinguishment of Debt	(4,453,000)	(1,494,000)	(89,883,000)	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 Mortgage Loans Accounted For Fair Value	5	6
Outstanding balance of mortgage loans accounted for at fair value	951,500,000	953,100,000						234,400,000
Interest Rate Used To Account For Mortgage Loan At Fair Value Upon Assumption								2.56%
nterest Expense Decrease Due To Recording Mortgage Loans At Fair Value	7,000,000	9,200,000	3,800,000
Cumulative Liability Related to the Fair Value of Debt Adjustment	$ 38,600,000	$ 23,800,000