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Significant Accounting Policies - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 Financial_Institution	Jun. 30, 2012	Jun. 30, 2013 Segment Financial_Institution	Jun. 30, 2012	Dec. 31, 2012
Accounting Policies [Abstract]
Cash and cash equivalents maturity period minimum			90 days
Number of financial institutions	1		1
Accounts receivable related-party	$ 12,326		$ 12,326		$ 8,180
ETF advisory fee revenue	37,101	20,230	66,254	39,205
Marketing fees	$ 428	$ 82	$ 539	$ 632
Number of operating segments			1

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For an unclassified balance sheet, amount of receivables arising from transactions with related parties.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

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-Subparagraph (SX 210.4-08.(k)(1))

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-Name Regulation S-X (SX)

-Number 210

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-Name Regulation S-X (SX)

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Amount of fees earned from providing investment advice and research to customers.

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Expenditures for planning and executing the conception, pricing, promotion, and distribution of ideas, goods, and services. Costs of public relations and corporate promotions are typically considered to be marketing costs.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Cash and cash equivalents criteria maturity period.

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Number of financial institutions.

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