CONDENSED CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION UNAUDITED (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
GAMCO Investors, Inc. stockholders equity
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0	0	0
Preferred stock, shares outstanding (in shares)	0	0	0	0
Common Stock, outstanding (in shares)	29,800,000	29,800,000	29,800,000	25,700,000
Treasury stock, shares (in shares)	4,801,829	4,801,829	4,758,794	8,874,702
Class A [Member]
GAMCO Investors, Inc. stockholders equity
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	100,000,000	100,000,000	100,000,000	100,000,000
Common Stock, issued (in shares)	15,482,982	15,482,982	15,422,901	15,381,179
Common Stock, outstanding (in shares)	10,681,153	10,681,153	10,664,107	6,506,477
Class B [Member]
GAMCO Investors, Inc. stockholders equity
Common Stock, par value (in dollars per share)	$ 0.001	$ 0.001	$ 0.001	$ 0.001
Common Stock, shares authorized (in shares)	100,000,000	100,000,000	100,000,000	100,000,000
Common Stock, issued (in shares)	24,000,000	24,000,000	24,000,000	24,000,000
Common Stock, outstanding (in shares)	19,093,311	19,093,311	19,156,792	19,218,814
AC 4% PIK Note [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	4.00%	4.00%
Debt instrument, maturity date	Nov. 30, 2020	Nov. 30, 2020
Loan from GGCP [Member]
LIABILITIES AND EQUITY
Debt instrument, maturity date		Dec. 28, 2016
5.875% Senior Notes [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	5.875%	5.875%
Debt instrument, maturity date		Jun. 01, 2021
Zero Coupon Subordinated Debentures [Member]
LIABILITIES AND EQUITY
Debt instrument, interest rate	0.00%	0.00%
Debt instrument, maturity date		Dec. 31, 2015
Zero coupon subordinated debentures, face value	$ 0.0	$ 0.0	$ 0.0	$ 6.9

This element represents the face value of the Zero coupon subordinated debentures due December 31, 2015.

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Face amount or stated value per share of common stock.

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The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

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Contractual interest rate for funds borrowed, under the debt agreement.

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Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

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The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

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Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

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Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

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Number of common and preferred shares that were previously issued and that were repurchased by the issuing entity and held in treasury on the financial statement date. This stock has no voting rights and receives no dividends.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29,30)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

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