Related Party Transactions - Income from its non-voting revenues interest and its non-voting profits interests in EAM (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Oct. 31, 2013	Oct. 31, 2012	Oct. 31, 2013	Oct. 31, 2012
Related Party Transactions [Abstract]
Non-voting revenues interest in EAM	$ 1,663	$ 1,414	$ 3,261	$ 2,808
Non-voting profits interest in EAM	191	115	362	194
Revenues and profits interests in EAM Trust	$ 1,854	$ 1,529	$ 3,623	$ 3,002