Derivative Instruments and Hedging Activities (Details) (USD $)	Apr. 30, 2013	Apr. 30, 2012
Other Assets [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	$ 18,970,000	$ 28,000,000
Derivative Asset, Fair Value, Gross Asset	946,000	965,000
Other Assets [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	618,824,000	638,762,000
Derivative Asset, Fair Value, Gross Asset	19,377,000	20,349,000
Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	1,258,045,000	1,258,315,000
Derivative Liability, Fair Value, Gross Liability	139,542,000	140,437,000
Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	39,452,000	39,562,000
Derivative Liability, Fair Value, Gross Liability	16,151,000	15,287,000
Interest rate caps [Member] | Other Assets [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Interest rate caps [Member] | Other Assets [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	478,024,000	479,085,000
Derivative Asset, Fair Value, Gross Asset	18,000	7,000
Interest rate caps [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Interest rate caps [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Interest rate swap agreements [Member] | Other Assets [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	0	0
Derivative Asset, Fair Value, Gross Asset	0	0
Interest rate swap agreements [Member] | Other Assets [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	0	18,877,000
Derivative Asset, Fair Value, Gross Asset	0	241,000
Interest rate swap agreements [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	1,019,650,000	1,019,920,000
Derivative Liability, Fair Value, Gross Liability	128,552,000	129,522,000
Interest rate swap agreements [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	0	0
Derivative Liability, Fair Value, Gross Liability	0	0
Total Return Swap [Member] | Other Assets [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	18,970,000	28,000,000
Derivative Asset, Fair Value, Gross Asset	946,000	965,000
Total Return Swap [Member] | Other Assets [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivatives Asset, Notional Amount	140,800,000	140,800,000
Derivative Asset, Fair Value, Gross Asset	19,359,000	20,101,000
Total Return Swap [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	238,395,000	238,395,000
Derivative Liability, Fair Value, Gross Liability	10,990,000	10,915,000
Total Return Swap [Member] | Accounts Payable, Accrued Expenses and Other Liabilities [Member] | Not Designated as Hedging Instrument [Member]
Fair Value of Derivative Instruments
Derivative Liability, Notional Amount	39,452,000	39,562,000
Derivative Liability, Fair Value, Gross Liability	$ 16,151,000	$ 15,287,000