Consolidated Sponsored Investment Products - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	$ 236,652	$ 139,054
Liabilities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	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	191
Liabilities [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	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	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	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	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	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	0
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	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	237,033	139,054
Assets [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	136,115	47,114
Assets [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	100,537	91,940
Assets [Member] | Derivatives Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	381
Assets [Member] | Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	82,571	91,940
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	82,417	91,940
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	154
Assets [Member] | Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	153,397	47,114
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	135,050	47,114
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	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	227
Assets [Member] | Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	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	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