CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Apr. 30, 2013	Apr. 30, 2012
Other assets
Receivable arising from straight-lining of rents, allowance	$ 830	$ 1,209
Accounts receivable, allowance	563	154
Intangible assets, accumulated amortization	27,708	47,813
Property and equipment, accumulated depreciation	1,673	1,423
Deferred charges and leasing costs, accumulated amortization	18,714	16,244
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)	101,487,976	89,473,838
Common Shares of Beneficial Interest, shares outstanding (in shares)	101,487,976	89,473,838
Noncontrolling interests - Operating Partnership (in shares)	21,635,127	20,332,415
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