Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 48,620	$ 54,772
Total assets measured at fair value	129,335	101,578
Sponsored funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	60,019	29,331
Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	11,473	9,168
Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	3,415	2,997
Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	5,808	5,310
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	48,620	54,772
Total assets measured at fair value	129,335	101,578
Level 1 [Member] | Sponsored funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	60,019	29,331
Level 1 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	11,473	9,168
Level 1 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	3,415	2,997
Level 1 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	5,808	5,310
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total assets measured at fair value	0	0
Level 2 [Member] | Sponsored funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 2 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 2 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	0	0
Level 2 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	0	0
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total assets measured at fair value	0	0
Level 3 [Member] | Sponsored funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 3 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0	0
Level 3 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	0	0
Level 3 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	$ 0	$ 0