Land held for divestiture (Details Textuals) (USD $)	3 Months Ended			6 Months Ended
	Jul. 31, 2012	Apr. 30, 2012	Jul. 31, 2011	Jul. 31, 2012	Jul. 31, 2011
Land held for divestiture [Line Items]
Ownership percentage of land development opportunity		100.00%
Impairment of land held for divestiture	$ 15,796,000		$ 0	$ 15,796,000	$ 0
Prosper, Texas [Member]
Land held for divestiture [Line Items]
Partnership interests	51.00%			51.00%
Sales price of land	29,800,000			29,800,000
Gain loss on sale of land	7,600,000			7,600,000
Gain loss on sales of land net of noncontrolling interest	3,900,000			3,900,000
Central Station [Member]
Land held for divestiture [Line Items]
Unsolicited offer to purchase ownership interest in remaining land parcels	30,000,000
Impairment of land held for divestiture	17,000,000			17,000,000
Mesa del Sol [Member]
Land held for divestiture [Line Items]
Impairment of land held for divestiture	$ 15,000,000			$ 15,000,000
Acres of land held for divestiture	3,000			3,000