Consolidated Sponsored Investment Products - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	$ 207,279	$ 139,054
Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	112,783	47,114
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	94,496	91,940
Derivatives [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	46
Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	11,044
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	93,347	91,940
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
Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	93,332	91,940
Level 1 [Member] | Derivatives [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	15
Level 1 [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	9,024
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	113,528	47,114
Level 2 [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	112,333	47,114
Level 2 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	1,164
Level 2 [Member] | Derivatives [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	31
Level 2 [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	2,020
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	450
Level 3 [Member] | Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	450
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
Level 3 [Member] | Derivatives [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	0
Level 3 [Member] | Short Sales [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total Investments of Consolidated Sponsored Investment Products at Fair Value	$ 0