Related Party Transactions (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2012	Oct. 31, 2011	Oct. 31, 2012	Oct. 31, 2011
Schedule of Equity Method Investments [Line Items]
Non-voting revenues interest in EAM			$ 2,808	$ 2,821
Non-voting profits interest in EAM			194	94
Revenues and profits interests in EAM Trust	1,529	1,343	3,002	2,915
EAM Trust
Schedule of Equity Method Investments [Line Items]
Non-voting revenues interest in EAM	1,414	1,330	2,808	2,821
Non-voting profits interest in EAM	115	13	194	94
Revenues and profits interests in EAM Trust	$ 1,529	$ 1,343	$ 3,002	$ 2,915