Consolidated Sponsored Investment Products - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	$ 126,056	$ 257,368
Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	126,056	257,368
Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	0	0
Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets measured at fair value	0	0
Consolidated Sponsored Investment Products [Member]
Assets, Fair Value Disclosure [Abstract]
Derivatives	3,279
Liabilities, Fair Value Disclosure [Abstract]
Derivatives	1,843
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Derivatives	758	381
Total assets measured at fair value	312,187	237,033
Liabilities, Fair Value Disclosure [Abstract]
Derivatives	1,320	191
Short sales	9,855	8,165
Total liabilities measured at fair value	11,175	8,356
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	148,589	136,115
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Equity Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	162,840	100,537
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Derivatives	125	154
Total assets measured at fair value	155,888	82,571
Liabilities, Fair Value Disclosure [Abstract]
Derivatives	403	191
Short sales	9,432	7,491
Total liabilities measured at fair value	9,835	7,682
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	0	0
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Equity Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	155,763	82,417
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Derivatives	633	227
Total assets measured at fair value	155,373	153,397
Liabilities, Fair Value Disclosure [Abstract]
Derivatives	917	0
Short sales	423	674
Total liabilities measured at fair value	1,340	674
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	147,663	135,050
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Equity Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	7,077	18,120
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Derivatives	0	0
Total assets measured at fair value	926	1,065
Liabilities, Fair Value Disclosure [Abstract]
Derivatives	0	0
Short sales	0	0
Total liabilities measured at fair value	0	0
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	926	1,065
Consolidated Sponsored Investment Products [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Equity Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Investments	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value of obligations incurred from short sales.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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-Glossary Trading Securities

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Fair value of financial and nonfinancial obligations.

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