Consolidation of Sponsored Investment Products - Consolidated Statement of Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Statements, Captions [Line Items]
Current assets	$ 461,138	$ 164,826
Non-current assets	140,496	167,923
Total assets	601,634	332,749
Current liabilities	108,903	62,411
Non-current liabilities	8,072	22,704
Total liabilities	116,975	85,115
Redeemable noncontrolling interest	18,855	3,163
Equity attributable to stockholders of the Company	465,853	244,474	183,155
Non-redeemable noncontrolling interest	(49)	(3)
Total liabilities and equity	601,634	332,749
Reportable Legal Entities [Member] \| Balance Before Consolidation of Investment Products [Member]
Condensed Financial Statements, Captions [Line Items]
Current assets	399,485	161,286
Non-current assets	140,496	167,923
Total assets	539,981	329,209
Current liabilities	66,105	62,034
Non-current liabilities	8,072	22,704
Total liabilities	74,177	84,738
Redeemable noncontrolling interest
Equity attributable to stockholders of the Company	465,853	244,474
Non-redeemable noncontrolling interest	(49)	(3)
Total liabilities and equity	539,981	329,209
Reportable Legal Entities [Member] \| Consolidated Investment Products [Member]
Condensed Financial Statements, Captions [Line Items]
Current assets	139,044	43,924
Non-current assets
Total assets	139,044	43,924
Current liabilities	42,824	364
Non-current liabilities
Total liabilities	42,824	364
Redeemable noncontrolling interest
Equity attributable to stockholders of the Company	96,220	43,560
Non-redeemable noncontrolling interest
Total liabilities and equity	139,044	43,924
Consolidation, Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Current assets	(77,391)	(40,384)
Non-current assets
Total assets	(77,391)	(40,384)
Current liabilities	(26)	13
Non-current liabilities
Total liabilities	(26)	13
Redeemable noncontrolling interest	18,855	3,163
Equity attributable to stockholders of the Company	(96,220)	(43,560)
Non-redeemable noncontrolling interest
Total liabilities and equity	$ (77,391)	$ (40,384)

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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