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	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Related Party Transaction [Line Items]
Non-voting revenues interest in EAM	$ 5,781	$ 5,684	$ 2,187
Non-voting profits interest in EAM	479	206	168
Revenues and profits interests in EAM Trust	6,260	5,890	2,355
EAM Trust
Related Party Transaction [Line Items]
Non-voting revenues interest in EAM	5,781	5,684	2,187	[1]
Non-voting profits interest in EAM	479	206	168
Revenues and profits interests in EAM Trust	$ 6,260	$ 5,890	$ 2,355

[1] Total results of EAM's investment management operations was for the period from December 23, 2010 through April 30, 2011.