Consolidated Sponsored Investment Products - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	$ 270,997	$ 236,652
Liabilities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	10,129	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	328	191
Liabilities [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	9,801	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	9,835	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	262	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	9,573	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	294	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	66
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	228	674
Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	271,130	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	148,134	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	122,863	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	133	381
Assets [Member] | Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	122,548	82,571
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	122,443	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	105	154
Assets [Member] | Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	147,730	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	147,282	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	420	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	28	227
Assets [Member] | Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	852	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	$ 852	$ 1,065