Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 223,087	$ 48,620
Total assets measured at fair value	299,868	129,335
Sponsored mutual funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	54,998	60,019
Equity securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	11,974	11,473
Sponsored closed-end funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	3,623	3,415
Nonqualified retirement plan assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	6,186	5,808
Level 1
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	223,087	48,620
Total assets measured at fair value	299,868	129,335
Level 1 | Sponsored mutual funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	54,998	60,019
Level 1 | Equity securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	11,974	11,473
Level 1 | Sponsored closed-end funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	3,623	3,415
Level 1 | Nonqualified retirement plan assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	6,186	5,808
Level 2
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total assets measured at fair value	0	0
Level 2 | Sponsored mutual funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	0	0
Level 2 | Equity securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	0	0
Level 2 | Sponsored closed-end funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	0	0
Level 2 | Nonqualified retirement plan assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	0	0
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0	0
Total assets measured at fair value	0	0
Level 3 | Sponsored mutual funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	0	0
Level 3 | Equity securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	0	0
Level 3 | Sponsored closed-end funds
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available-for-sale	0	0
Level 3 | Nonqualified retirement plan assets
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	$ 0	$ 0