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Significant Accounting Policies - Revenues and Expenses under Collaborative Arrangement (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Related Party Transaction [Line Items]        
ETF advisory fee revenue $ 37,101 $ 20,230 $ 66,254 $ 39,205
Expenses:        
Fund management and administration 9,106 5,567 17,329 11,006
Marketing and advertising 2,196 1,548 4,133 2,874
Sales and business development 1,520 842 3,321 1,702
Total expenses 25,089 20,264 46,576 38,319
Net income 12,242 129 20,096 1,244
Third-Party Sharing Arrangements [Member]
        
Related Party Transaction [Line Items]        
ETF advisory fee revenue   2,733   5,523
Expenses:        
Fund management and administration   335   676
Marketing and advertising   9   19
Sales and business development   6   10
Total expenses   350   705
Net income   2,383   4,818
Sharing   $ 1,147   $ 2,342