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Consolidated Sponsored Investment Products - Additional Information (Detail)
6 Months Ended 12 Months Ended
Jun. 30, 2015
USD ($)
Product
Dec. 31, 2014
Product
Jun. 30, 2014
USD ($)
Condensed Financial Statements, Captions [Line Items]      
Number of consolidated sponsored investment products 12 12  
Number of additional consolidated sponsored investment product 1    
Number of deconsolidated sponsored investment products 1    
Fair value, securities, Level 2 to Level 1 transfers, amount | $ $ 14,600,000   $ 0
Fair value, securities, Level 1 to Level 2 transfers, amount | $ $ 0   $ 0
Maximum [Member]      
Condensed Financial Statements, Captions [Line Items]      
Percentage of permitted borrowings 33.33%