Consolidated Sponsored Investment Products - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Product	Dec. 31, 2013 Product	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Number of consolidated sponsored investment products	12	8
Number of additional consolidated sponsored investment products	6
Number of deconsolidated sponsored investment products	2
Fair value, securities, Level 1 to Level 2 transfers, amount	$ 0	$ 0	$ 0
Percentage of permitted borrowings	33.33%
Securities [Member]
Condensed Financial Statements, Captions [Line Items]
Fair value, securities, Level 1 to Level 2 transfers, amount	$ 1,500,000