CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Jan. 31, 2014	Apr. 30, 2013
Other assets
Receivable arising from straight-lining of rents, allowance	$ 783,000	$ 830,000
Accounts receivable, allowance	260,000	563,000
Intangible assets, accumulated amortization	22,703,000	27,708,000
Property and equipment, accumulated depreciation	1,938,000	1,673,000
Deferred charges and leasing costs, accumulated amortization	20,051,000	18,714,000
Statement [Line Items]
Common Shares of Beneficial Interest, no par value (in dollars per share)	$ 0	$ 0
Common Shares of Beneficial Interest, shares issued (in shares)	106,937,304	101,487,976
Common Shares of Beneficial Interest, shares outstanding (in shares)	106,937,304	101,487,976
Noncontrolling interests - Operating Partnership (in shares)	21,799,285	21,635,127
Preferred Class A [Member]
Statement [Line Items]
Preferred Shares of Beneficial Interest, no par value (in dollars per share)	$ 0	$ 0
Preferred Shares of Beneficial Interest, shares issued (in shares)	1,150,000	1,150,000
Preferred Shares of Beneficial Interest, shares outstanding (in shares)	1,150,000	1,150,000
Preferred Shares of Beneficial Interest, aggregate liquidation preference	28,750,000	28,750,000
Preferred Class B [Member]
Statement [Line Items]
Preferred Shares of Beneficial Interest, no par value (in dollars per share)	$ 0	$ 0
Preferred Shares of Beneficial Interest, shares issued (in shares)	4,600,000	4,600,000
Preferred Shares of Beneficial Interest, shares outstanding (in shares)	4,600,000	4,600,000
Preferred Shares of Beneficial Interest, aggregate liquidation preference	$ 115,000,000	$ 115,000,000