Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 54,772	$ 202,054
Other investments	5,310	5,063
Total assets measured at fair value	101,578	257,368
Sponsored funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	29,331	38,312
Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	9,168	8,740
Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	2,997	3,199
Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	5,310	5,063
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	54,772	202,054
Total assets measured at fair value	101,578	257,368
Level 1 [Member] | Sponsored funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	29,331	38,312
Level 1 [Member] | Equity securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	9,168	8,740
Level 1 [Member] | Sponsored closed-end funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	2,997	3,199
Level 1 [Member] | Nonqualified retirement plan assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments	5,310	5,063
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