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Variable Interests (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Apr. 30, 2011
Apr. 30, 2010
Management fees revenue $ 4.1 $ 22.6
Percent of managing directors as partners 84.00%  
Aggregate percent of compensation payable 67.00%  
Contributions to the retirement plan 60.0  
Maximum exposure to economic loss $ 94.8  
M&P [Member]
   
Ownership interest in partnership 100.00%