Discontinued Operations (Details) (USD $)	12 Months Ended									0 Months Ended		12 Months Ended				0 Months Ended		12 Months Ended			0 Months Ended					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Three non-core strip shopping centers adn two DC office buildings [Member]	Dec. 31, 2010 Cannery and Pentagon Row [Member]	Dec. 31, 2012 Retail Properties Segment [Member]	Dec. 31, 2011 Retail Properties Segment [Member]	Dec. 31, 2010 Retail Properties Segment [Member]	Dec. 31, 2012 Retail Properties Segment [Member] Retail Segment Strip Shopping Centers [Member] Properties	Feb. 13, 2013 Retail Properties Segment [Member] Plant [Member] Subsequent Event [Member]	Jan. 24, 2013 Retail Properties Segment [Member] Green Acres Mall [Member] Retail Segment Regional Malls [Member] Subsequent Event [Member]	Dec. 31, 2012 Retail Properties Segment [Member] Market Street [Member] Subsequent Event [Member]	Dec. 31, 2012 Washington DC Segment [Member]	Dec. 31, 2011 Washington DC Segment [Member]	Dec. 31, 2010 Washington DC Segment [Member]	Jul. 26, 2012 Washington DC Segment [Member] 409 Third Street SW [Member] sqft	Nov. 07, 2012 Washington DC Segment [Member] Reston Executive [Member] Properties sqft	Dec. 31, 2012 Merchandise Mart Segment [Member]	Dec. 31, 2011 Merchandise Mart Segment [Member]	Dec. 31, 2010 Merchandise Mart Segment [Member]	Jan. 06, 2012 Merchandise Mart Segment [Member] West Mart Center 350 [Member] sqft	Jun. 22, 2012 Merchandise Mart Segment [Member] LA Mart [Member] sqft	Jul. 26, 2012 Merchandise Mart Segment [Member] Washington Design Center and Canadian Trade Shows [Member]	Jul. 26, 2012 Merchandise Mart Segment [Member] Washington Design Center [Member] sqft	Jul. 26, 2012 Merchandise Mart Segment [Member] Canadian Trade Shows [Member]	Dec. 31, 2012 Merchandise Mart Segment [Member] Boston Design Center [Member] sqft	Dec. 31, 2011 Merchandise Mart Segment [Member] High Point Complex [Member]	Dec. 31, 2012 Other Segment [Member]	Dec. 31, 2011 Other Segment [Member]	Dec. 31, 2010 Other Segment [Member]
Disposal Group Including Discontinued Operation Additional Disclosures Abstract
Number of real estate properties sold									12								3
Square footage of real estate property sold																409,000	494,000				1,200,000	784,000		393,000		554,000
Gross proceeds from the sale of real estate											$ 500,000,000						$ 126,250,000					$ 53,000,000
Cash proceeds from the sale of real estate				168,000,000	14,992,000				157,000,000							200,000,000					228,000,000	18,000,000	103,000,000			72,400,000
Net proceeds from the sale of real estate											185,000,000
Anticipated gross proceeds from the sale of real estate										203,000,000		60,000,000
Anticipated net proceeds from the sale of real estate										93,000,000
Anticipated net gain on sale of real estate										33,000,000		35,000,000
Seller financing rate																						6.00%
Seller Financing Related To Property Sales	35,000,000	0	0																			35,000,000
Anticipated tax gain on sale of real estate											304,000,000
Income tax on gain on sale	11,448,000
Discontinued Operation, Income (Loss) from Discontinued Operations Disclosure [Abstract]
Total revenues	147,404,000	230,314,000	267,008,000
Total expenses	102,479,000	175,930,000	227,626,000
Income from discontinued operations before gain on sale of real estate	44,925,000	54,384,000	39,382,000
Net gains on sale of real estate	245,799,000	51,623,000	2,506,000	51,623,000					22,266,000		205,000,000					126,621,000	36,746,000				54,911,000					5,252,000
Gain on sale of Canadian Trade Shows	19,657,000	0	0																						19,657,000
Impairment losses and litigation loss accrual	(24,439,000)	(28,799,000)	(35,056,000)
Net gain on extinguishment of debt		83,907,000																									83,907,000
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total	285,942,000	161,115,000	6,832,000			42,926,000	31,815,000	19,061,000					167,766,000	52,390,000	4,143,000			75,144,000	72,971,000	(20,948,000)								747,000	3,376,000	4,408,000
Disposal Group Including Discontinued Operation Unclassified Balance Sheet Disclosures Abstract
Assets related to discontinued operations	374,476,000	1,049,643,000				340,977,000	474,402,000							152,568,000				7,759,000	385,381,000									25,740,000	37,292,000
Liabilities related to discontinued operations	$ 315,448,000	$ 506,960,000				$ 315,448,000	$ 339,724,000							$ 93,000,000					$ 74,236,000