Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value	$ 67,213	$ 61,245
Sponsored mutual funds and variable insurance funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	13,371	8,013
Investments of consolidated sponsored investment products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	43,114	43,227
Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	4,649	4,379
Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	2,713	2,656
Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other assets, nonqualified retirement plan assets	3,366	2,970
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value	37,748	28,110
Level 1 [Member] | Sponsored mutual funds and variable insurance funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	13,371	8,013
Level 1 [Member] | Investments of consolidated sponsored investment products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	13,649	10,092
Level 1 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	4,649	4,379
Level 1 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	2,713	2,656
Level 1 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other assets, nonqualified retirement plan assets	3,366	2,970
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value	29,465	33,135
Level 2 [Member] | Sponsored mutual funds and variable insurance funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading
Level 2 [Member] | Investments of consolidated sponsored investment products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	29,465	33,135
Level 2 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading
Level 2 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale
Level 2 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other assets, nonqualified retirement plan assets
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value
Level 3 [Member] | Sponsored mutual funds and variable insurance funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading
Level 3 [Member] | Investments of consolidated sponsored investment products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading
Level 3 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading
Level 3 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale
Level 3 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other assets, nonqualified retirement plan assets