Fair Value Measurement (Assets Remeasured At Fair Value On Recurring Basis) (Details) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total, Gain (loss) on financial instruments	$ (4,666)	$ 6,389
Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets at fair value	465,344	389,654
As a percentage of total assets	14.20%	10.00%
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets at fair value	0	0
As a percentage of total assets	0.00%	0.00%
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets at fair value	465,344	389,654
As a percentage of total assets	14.20%	10.00%
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets at fair value	0	0
As a percentage of total assets	0.00%	0.00%
Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on investments	(4,926)	6,007
Mortgage-Backed Securities | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	460,935	384,062
Mortgage-Backed Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Mortgage-Backed Securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	460,935	384,062
Mortgage-Backed Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain on investments	260	382
Municipal Bonds | Total
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,409	5,592
Municipal Bonds | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	0	0
Municipal Bonds | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,409	5,592
Municipal Bonds | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 0	$ 0