CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	Oct. 31, 2012	Apr. 30, 2012
Other assets
Receivable arising from straight-lining of rents, allowance	$ 1,310	$ 1,209
Accounts receivable, allowance	410	154
Intangible assets, accumulated amortization	25,579	47,813
Property and equipment, accumulated depreciation	1,518	1,423
Deferred charges and leasing costs, accumulated amortization	16,484	16,244
Statement [Line Items]
Noncontrolling interests - Operating Partnership (in shares)	21,336,222	20,332,415
Common Shares of Beneficial Interest, no par value (in dollars per share)	$ 0	$ 0
Common Shares of Beneficial Interest, shares issued (in shares)	93,161,087	89,473,838
Common Shares of Beneficial Interest, shares outstanding (in shares)	93,161,087	89,473,838
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	0
Preferred Shares of Beneficial Interest, shares outstanding (in shares)	4,600,000	0
Preferred Shares of Beneficial Interest, aggregate liquidation preference	$ 115,000,000	$ 115,000,000