Fair Value Measurements - Changes in Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	$ 202,054	$ 270,262
Total assets measured at fair value	257,368us-gaap_AssetsFairValueDisclosure	303,450us-gaap_AssetsFairValueDisclosure
Other investments, nonqualified retirement plan assets	5,063	4,220
Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	38,312	17,904
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	8,740	8,379
Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	3,199	2,685
Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	5,063	4,220
Level 1 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Cash equivalents	202,054	270,262
Total assets measured at fair value	257,368	303,450
Level 1 [Member] \| Sponsored Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	38,312	17,904
Level 1 [Member] \| Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities trading	8,740	8,379
Level 1 [Member] \| Sponsored Closed-End Funds [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Marketable securities available for sale	3,199	2,685
Level 1 [Member] \| Nonqualified Retirement Plan Assets [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Other investments, nonqualified retirement plan assets	5,063	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
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 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

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

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- Definition

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

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- Definition

Investments In Nonqualified Retirement Plan Assets

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