Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 121,552	$ 54,772
Total assets measured at fair value	207,216	101,578
Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	66,465	29,331
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	10,200	9,168
Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	3,569	2,997
Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	5,430	5,310
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	121,552	54,772
Total assets measured at fair value	207,216	101,578
Level 1 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	66,465	29,331
Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	10,200	9,168
Level 1 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	3,569	2,997
Level 1 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	5,430	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, nonqualified retirement plan assets	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, nonqualified retirement plan assets	$ 0	$ 0