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Significant Accounting Policies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Financial_Institution	Sep. 30, 2012	Sep. 30, 2013 Segment Financial_Institution	Sep. 30, 2012	Dec. 31, 2012
Related Party Transaction [Line Items]
Cash and cash equivalents maturity period maximum			90 days
Number of financial institutions	1		1
Accounts receivable related-party	$ 12,977		$ 12,977		$ 8,180
ETF advisory fee revenue	39,437	21,440	105,691	60,645
Marketing fees	374	81	913	713
Number of operating segments			1
Leasehold improvements [Member]
Related Party Transaction [Line Items]
Accounts receivable	$ 3,223		$ 3,223