CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Jul. 31, 2014	Apr. 30, 2014
Other assets
Receivable arising from straight-lining of rents, allowance	$ 717,000	$ 796,000
Accounts receivable, allowance	306,000	248,000
Intangible assets, accumulated amortization	25,497,000	24,071,000
Property and equipment, accumulated depreciation	2,140,000	2,041,000
Deferred charges and leasing costs, accumulated amortization	22,237,000	21,068,000
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)	114,762,774	109,019,341
Common Shares of Beneficial Interest, shares outstanding (in shares)	114,762,774	109,019,341
Noncontrolling interests - Operating Partnership (in shares)	17,975,440	21,093,445
Preferred Series 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 Series 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