Real Estate (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended						10 Months Ended	12 Months Ended		0 Months Ended	10 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended	0 Months Ended			3 Months Ended	0 Months Ended			12 Months Ended	0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Oct. 04, 2012 Fountain Square [Member] sqft	Jan. 03, 2012 Two Patriots Park [Member] sqft	Dec. 31, 2012 Office Technical Properties [Member] Real_Estate_Properties	May 04, 2012 One Patriots Park [Member] sqft	Apr. 30, 2012 510 Madison Avenue [Member] sqft	Dec. 31, 2012 Class A Office Properties [Member] Real_Estate_Properties	Dec. 31, 2012 453 Ravendale Drive [Member]	Dec. 31, 2012 453 Ravendale Drive [Member]	Mar. 01, 2012 453 Ravendale Drive [Member] sqft	Mar. 13, 2012 100 Federal Street [Member] sqft floor	Dec. 31, 2012 100 Federal Street [Member]	Dec. 31, 2012 100 Federal Street [Member]	Oct. 04, 2012 Fountain Square [Member]	Dec. 31, 2012 Fountain Square [Member]	Aug. 29, 2012 680 Folsom Street [Member] sqft	Aug. 29, 2012 Six Nine Zero Folsom Street [Member] floor sqft	Jan. 31, 2012 Montvale Center [Member]	Mar. 31, 2012 Montvale Center [Member]	May 17, 2012 Bedford Business Park [Member] sqft	May 17, 2012 Bedford Business Park [Member] Office Technical Properties [Member] Buildings	May 17, 2012 Bedford Business Park [Member] Class A Office Properties [Member] Buildings	Dec. 31, 2012 series four preferred units [Member]	Aug. 29, 2012 series four preferred units [Member] 680 Folsom Street [Member]	Oct. 04, 2012 Retail Properties [ Member] Fountain Square [Member] sqft	Oct. 04, 2012 Class A Office Properties [Member] Fountain Square [Member] sqft
Real Estate Properties [Line Items]
Coupon/Stated Rate				5.71%
Companys Joint Venture Ownership Percentage				50.00%
Acquisition date											Mar. 01, 2012				Mar. 13, 2012			Aug. 29, 2012
Acquisition aggregate purchase price				$ 385,000,000								$ 6,700,000	$ 615,000,000			$ 410,016,000
Noncash or Part Noncash Acquisition, Debt Assumed				211,300,000												211,250,000
Partners Joint Venture Ownership Percentage				50.00%
Square footage of signed lease (in square feet)													732,000
Number of floors in building													37						2
Consideration paid				87,000,000														62,200,000
Number of Series Four Units Issued (in units)																		1,588,100
Preferred Units liquidation preference (in dollars per share)																										$ 50.00
Quarterly Distributions to Series Four Preferred Units																									$ 0.25	$ 0.25
Series Four Preferred Units Annual Dividend Payable Rate																									2.00%	2.00%
Construction Loan Facility Borrowing Capacity																		170,000,000
Revenues contributed by acquisitions	1,876,267,000	1,751,320,000	1,543,310,000							500,000				52,600,000			8,700,000
Operating income	651,681,000	616,759,000	609,072,000							200,000				8,500,000			(3,200,000)
Debt default, accrued interest rate																				9.93%
Maturity date				Oct. 11, 2016																Jun. 06, 2012
Gain on forgiveness of debt from discontinued operations																					17,800,000
Disposition Price																						62,800,000
Proceeds from the sale of real estate	61,963,000	0	0																			62,000,000
Gain on sale of real estate from discontinued operations	36,877,000	0	0																			36,900,000
Number of real estate properties						17			132														2	1
Net Rentable Area				758,000	256,000		268,000	356,000				30,000	1,265,000					522,000	22,000			470,000					237,000	521,000
Percentage of Project Pre Leased																		85.00%
Estimated Project Costs																		340,000,000
Future Fixed Price to Acquire Remaining Portion of Joint venture				$ 102,000,000
Fixed Price Option Expiration Date				Jan. 31, 2016