Discontinued Operations (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended					0 Months Ended			6 Months Ended	0 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Merchandise Mart Segment [Member]	Jun. 30, 2011 Merchandise Mart Segment [Member]	Jun. 30, 2012 Merchandise Mart Segment [Member] Properties	Jun. 30, 2011 Merchandise Mart Segment [Member]	Dec. 31, 2011 Merchandise Mart Segment [Member]	Jun. 30, 2012 Washington DC Segment [Member]	Jun. 30, 2011 Washington DC Segment [Member]	Jun. 30, 2012 Washington DC Segment [Member] Properties	Jun. 30, 2011 Washington DC Segment [Member]	Jun. 30, 2012 Retail Properties Segment [Member]	Jun. 30, 2011 Retail Properties Segment [Member]	Jun. 30, 2012 Retail Properties Segment [Member] Properties	Jun. 30, 2011 Retail Properties Segment [Member]	Dec. 31, 2011 Retail Properties Segment [Member]	Jun. 30, 2012 409 Third Street SW [Member]	Dec. 31, 2011 409 Third Street SW [Member]	Jul. 26, 2012 409 Third Street SW [Member] Washington DC Segment [Member] Subsequent Event [Member] sqft	Jan. 06, 2012 West Mart 350 [Member] Merchandise Mart Segment [Member] sqft	Jun. 22, 2012 LA Mart [Member] Merchandise Mart Segment [Member] sqft	Jun. 30, 2012 LA Mart [Member] Merchandise Mart Segment [Member]	Jul. 05, 2012 Washington Design Center, Boston Design Center, and Canadian Trade Shows [Member] Merchandise Mart Segment [Member] Subsequent Event [Member]
Disposal Group Including Discontinued Operation Additional Disclosures Abstract
Number of real estate properties sold or agreed to sell								5					1				11
Disposal group including discontinued operation sales price of real estate			$ 792,000,000
Gross proceeds from the sale of real estate																								53,000,000
Cash proceeds from the sale of real estate																	136,000,000					200,000,000	228,000,000	18,000,000
Anticipated cash proceeds from real estate agreed to sell																										175,000,000
Square footage of real estate property sold																						409,000	1,200,000	784,000
Anticipated net gain on sales of real estate																										24,500,000
Seller financing rate																									6.00%
Seller Financing Related To Property Sales			35,000,000	0																				35,000,000
Discontinued Operation, Income (Loss) from Discontinued Operation Disclosures [Abstract]
Total revenues	22,678,000	34,509,000	49,429,000	76,622,000
Total expenses	14,051,000	24,598,000	33,444,000	59,951,000
Income from discontinued operations before gain on sale of real estate	8,627,000	9,911,000	15,985,000	16,671,000
Net gain on sales of real estate	16,896,000	458,000	72,713,000	51,623,000													17,802,000					124,000,000	54,911,000
Impairment losses	13,511,000		13,511,000
Net gain on extinguishment of debt				83,907,000
Income (Loss) from Discontinued Operations	12,012,000	10,369,000	75,187,000	152,201,000		(9,588,000)	3,294,000	47,499,000	87,882,000		3,713,000	2,490,000	5,943,000	51,439,000	10,744,000	4,593,000	15,395,000	12,890,000
Disposal Group Including Discontinued Operation Unclassified Balance Sheet Disclosures Abstract
Assets related to discontinued operations	301,946,000		301,946,000		661,724,000	134,698,000		134,698,000		376,571,000					102,620,000		102,620,000		220,249,000	64,628,000	64,904,000
Liabilities related to discontinued operations	$ 70,844,000		$ 70,844,000		$ 93,603,000	$ 67,071,000		$ 67,071,000		$ 74,236,000					$ 3,773,000		$ 3,773,000		$ 19,367,000	$ 0	$ 0