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Significant Accounting Policies - Revenues and Expenses under Collaborative Arrangement (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Related Party Transaction [Line Items]      
ETF advisory fee revenue $ 148,594 $ 84,024 $ 64,366
Expenses:      
Marketing and advertising 8,309 5,363 4,475
Sales and business development 6,474 3,586 3,603
Total expenses 97,931 73,768 62,068
Net income 51,537 11,030 3,092
Third-Party Sharing Arrangements [Member]
      
Related Party Transaction [Line Items]      
ETF advisory fee revenue   10,968 12,596
Expenses:      
Fund management and administration   1,341 1,712
Marketing and advertising   39 406
Sales and business development   24 159
Other     1
Total expenses   1,404 2,278
Net income   9,564 10,318
Sharing   $ 4,662 $ 5,114