Consolidated Sponsored Investment Products - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	$ 254,481	$ 236,652
Liabilities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	9,685	8,356
Liabilities [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	172	191
Liabilities [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	9,513	8,165
Level 1 [Member] | Liabilities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	9,104	7,682
Level 1 [Member] | Liabilities [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	129	191
Level 1 [Member] | Liabilities [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	8,975	7,491
Level 2 [Member] | Liabilities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	581	674
Level 2 [Member] | Liabilities [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	43
Level 2 [Member] | Liabilities [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	538	674
Level 3 [Member] | Liabilities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	0
Level 3 [Member] | Liabilities [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	0
Level 3 [Member] | Liabilities [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	0
Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	254,748	237,033
Assets [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	145,304	136,115
Assets [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	109,177	100,537
Assets [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	267	381
Assets [Member] | Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	108,496	82,571
Assets [Member] | Level 1 [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	0
Assets [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	108,408	82,417
Assets [Member] | Level 1 [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	88	154
Assets [Member] | Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	145,339	153,397
Assets [Member] | Level 2 [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	144,391	135,050
Assets [Member] | Level 2 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	769	18,120
Assets [Member] | Level 2 [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	179	227
Assets [Member] | Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	913	1,065
Assets [Member] | Level 3 [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	913	1,065
Assets [Member] | Level 3 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	0
Assets [Member] | Level 3 [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	$ 0