Net Gain on Disposition of Partial Interests in Rental Properties and Other Investment (Details Textuals) (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					3 Months Ended		6 Months Ended
	Jul. 31, 2011	Jul. 31, 2010	Jul. 31, 2011	Jul. 31, 2010	Jul. 31, 2011 New York Retail Joint Venture [Member]	Jul. 31, 2010 New York Retail Joint Venture [Member]	Jul. 31, 2011 New York Retail Joint Venture [Member]	Jul. 31, 2010 New York Retail Joint Venture [Member]	Mar. 29, 2011 New York Retail Joint Venture [Member]	Jul. 31, 2011 University Park Joint Venture [Member]	Jul. 31, 2010 University Park Joint Venture [Member]	Apr. 30, 2010 University Park Joint Venture [Member]	Jul. 31, 2011 University Park Joint Venture [Member]	Jul. 31, 2010 University Park Joint Venture [Member]	Feb. 22, 2010 University Park Joint Venture [Member]	Jul. 31, 2011 The Nets [Member]	Jul. 31, 2010 The Nets [Member]	Jul. 31, 2011 The Nets [Member]	Jul. 31, 2010 The Nets [Member]	May 12, 2010 The Nets [Member]	May 12, 2010 Arena [Member]	May 12, 2010 Atlantic Yards Development Company LLC [Member]	Jul. 31, 2011 Bernstein Joint Venture [Member]	Jul. 31, 2010 Bernstein Joint Venture [Member]	Jul. 31, 2011 Bernstein Joint Venture [Member]	Jul. 31, 2010 Bernstein Joint Venture [Member]	Feb. 19, 2010 Bernstein Joint Venture [Member]
Property Subject to or Available for Operating Lease [Line Items]
Number of properties held by newly formed joint venture									15						7												3
Percentage of ownership sold									49.00%						49.00%					80.00%	45.00%						50.00%
Number of retail properties wholly owned by the company									14
Percentage ownership of the company in partially owned retail property									75.00%
Nonrecourse mortgage debt on the properties assumed by the joint venture							$ 244,952,000					$ 320,000,000														$ 163,000,000
Net cash proceeds in the form of loan from the joint venture							178,286,000							135,117,000
Net gain on disposition of partial interests in rental properties and other investment	0	259,381,000	9,561,000	260,247,000	0	0	9,561,000	0		0	204,269,000		0	175,793,000		0	55,112,000	0	55,112,000				0	0	0	29,342,000
Deferred gains recorded in exchange of ownership interests					115,388,000		115,388,000					188,410,000
Transaction costs associated with gains received by the company in exchange of ownership interests							11,776,000				2,792,000	28,476,000
Participation payments made to the ground lessor of the seven properties							5,779,000				1,768,000	21,483,000
Outside partner's minimum initial investment requirement					20.00%		20.00%
Net cash proceeds received by the company in exchange of the ownership interest														140,545,000												28,922,000
Investment made by MP entities based on the terms of the purchase agreement																				223,000,000
Percentage of ownership right to acquire by the MP entities																						20.00%
Pre tax net gain recognized by the company pursuant to the purchase agreement with the MP entities																			55,112,000
Pre tax net gain after noncontrolling interest recognized by the company pursuant to the purchase agreement with the MP entities																			$ 31,437,000
Percentage of adjustment to fair value retained noncontrolling interest																			20.00%
Number of rental units																											1,340