Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 234,181	$ 270,262
Total assets measured at fair value	332,429	303,450
Sponsored Mutual Funds and Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	82,198	17,904
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	8,910	8,379
Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	2,841	2,685
Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	4,299	4,220
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	234,181	270,262
Total assets measured at fair value	332,429	303,450
Level 1 [Member] | Sponsored Mutual Funds and Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	82,198	17,904
Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	8,910	8,379
Level 1 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	2,841	2,685
Level 1 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	4,299	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 Mutual Funds and Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Level 2 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Level 2 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	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 Mutual Funds and Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Level 3 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	0
Level 3 [Member] | Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	0
Level 3 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	$ 0