Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 167,586	$ 270,262
Other investments, nonqualified retirement plan assets	4,546	4,220
Total assets measured at fair value	228,535	303,450
Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	43,981	17,904
Marketable securities available for sale	3,189	2,685
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	9,233	8,379
Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	4,546	4,220
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	167,586	270,262
Total assets measured at fair value	228,535	303,450
Level 1 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	43,981	17,904
Marketable securities available for sale	3,189	2,685
Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	9,233	8,379
Level 1 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	4,546	4,220
Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0
Total assets measured at fair value	0
Level 2 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Marketable securities available for sale	0
Level 2 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Level 2 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	0
Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	0
Total assets measured at fair value	0
Level 3 [Member] | Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Marketable securities available for sale	0
Level 3 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Level 3 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	$ 0