EQUITY (Details) (USD $)	6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Jan. 20, 2012
Class of Stock [Line Items]
Aggregate public offering price, maximum	$ 150,000,000
Aggregate gross sales price of common shares of beneficial interest, maximum			100,000,000
Proceeds from Issuance of Common Stock	2,448,000	330,000
Common shares issued under 2008 Incentive Plan Award (in shares)	53,000	53,000
Value of common shares issued under 2008 Incentive Plan Award	398,000,000	443,000,000
Units converted to common shares (in shares)	128,000,000	759,000,000
Units converted to common shares	579,000,000	3,500,000
Shares issued	2,846,000	773,000
Continuous equity offering program [Member]
Class of Stock [Line Items]
Shares issued, shares (in shares)	300,000,000	0
Proceeds from Issuance of Common Stock	2,100,000
401(k) plan [Member]
Class of Stock [Line Items]
Shares issued, shares (in shares)	43,000,000	46,000,000
Shares issued	337,000,000	330,000,000
Distribution Reinvestment and Share Purchase Plan [Member]
Class of Stock [Line Items]
Shares issued, shares (in shares)	3,200,000	2,300,000
Shares issued	24,400	16,900
Public Offering [Member]
Class of Stock [Line Items]
Shares per price, public offering (in dollars per share)	$ 25.00
Proceeds from issuance of Series B preferred shares	111,200,000
Per share amount of redeemable nonvoting stocks (in dollars per share)	$ 25
Per share amount of annual distributions accrued annually (in dollars per share)	$ 1.9875
Liquidation preference rate of preferred shares (in hundredths)	7.95%
Liquidation preference (in dollars per share)	$ 25
Amount of liquidation preference	115,000,000
Proceeds from issuance of Series B preferred units	4,600,000
Proceeds from issuance of Series B preferred units applied as debt repayment	$ 79,000,000
Shares issued, shares (in shares)	4.6