Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value	$ 61,245		$ 17,821
Affiliated Mutual Funds And Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	8,013		8,980
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	4,379		3,546
Affiliated Closed End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	2,656		2,469
Sponsored closed-end funds	2,656		2,469
Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Nonqualified retirement plan assets	2,970		2,826
Investment Of Consolidated Sponsored Investment Products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	43,227
Fair Value, Inputs, Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value	28,110		17,821
Fair Value, Inputs, Level 1 [Member] | Affiliated Mutual Funds And Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	8,013		8,980
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	4,379		3,546
Fair Value, Inputs, Level 1 [Member] | Affiliated Closed End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sponsored closed-end funds	2,656		2,469
Fair Value, Inputs, Level 1 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Nonqualified retirement plan assets	2,970		2,826
Fair Value, Inputs, Level 1 [Member] | Investment Of Consolidated Sponsored Investment Products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	10,092
Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value	33,135	[1]
Fair Value, Inputs, Level 2 [Member] | Affiliated Mutual Funds And Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities		[1]
Fair Value, Inputs, Level 2 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities		[1]
Fair Value, Inputs, Level 2 [Member] | Affiliated Closed End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sponsored closed-end funds		[1]
Fair Value, Inputs, Level 2 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Nonqualified retirement plan assets		[1]
Fair Value, Inputs, Level 2 [Member] | Investment Of Consolidated Sponsored Investment Products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities	33,135	[1]
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Total assets measured at fair value
Fair Value, Inputs, Level 3 [Member] | Affiliated Mutual Funds And Variable Insurance Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities
Fair Value, Inputs, Level 3 [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities
Fair Value, Inputs, Level 3 [Member] | Affiliated Closed End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Sponsored closed-end funds
Fair Value, Inputs, Level 3 [Member] | Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Nonqualified retirement plan assets
Fair Value, Inputs, Level 3 [Member] | Investment Of Consolidated Sponsored Investment Products [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Equity securities

[1] Debt securities are valued based on evaluated quotations received from independent pricing services or from dealers who make markets in such securities. For most bond types, the pricing service utilizes matrix pricing which considers one or more of the following factors: yield or price of bonds of comparable quality, coupon, maturity, current cash flows, type and current day trade information, as well as dealer supplied prices. These valuations are categorized as Level 2 in the hierarchy.