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		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Schedule of Equity Method Investments [Line Items]
Non-voting revenues interest in EAM			$ 4,280	$ 4,251
Non-voting profits interest in EAM			347	120
Revenues and profits interests in EAM Trust	1,625	1,456	4,627	4,371
EAM Trust
Schedule of Equity Method Investments [Line Items]
Non-voting revenues interest in EAM	1,472	1,430	4,280	4,251
Non-voting profits interest in EAM	153	26	347	120
Revenues and profits interests in EAM Trust	$ 1,625	$ 1,456	$ 4,627	$ 4,371