CONDENSED CONSOLIDATED BALANCE SHEETS (unaudited) (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Oct. 31, 2017	Apr. 30, 2017
LIABILITIES
Mortgages payable, loan costs	$ 3,062	$ 3,480
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract]
Common Shares of Beneficial Interest, no par value (in dollars per share)	$ 0	$ 0
Common Shares of Beneficial Interest, shares issued (in shares)	120,189	121,199
Common Shares of Beneficial Interest, shares outstanding (in shares)	120,189	121,199
Noncontrolling interests - Operating Partnership (in shares)	14,618	15,617
Series B Preferred Stock [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract]
Preferred Shares of Beneficial Interest, par value (in dollars per share)	$ 0	$ 0
Preferred Shares of Beneficial Interest, shares issued (in shares)	0	4,600
Preferred Shares of Beneficial Interest, shares outstanding (in shares)	0	4,600
Preferred shares liquidation preference	$ 115,000	$ 115,000
Series C Preferred Stock [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract]
Preferred Shares of Beneficial Interest, par value (in dollars per share)	$ 0	$ 0
Preferred Shares of Beneficial Interest, shares issued (in shares)	4,118
Preferred Shares of Beneficial Interest, shares outstanding (in shares)		0
Preferred shares liquidation preference	$ 102,962	$ 102,962