Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 31, 2020
Entity Information [Line Items]
Entity Registrant Name	GAMCO INVESTORS, INC. ET AL
Entity Central Index Key	0001060349
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Document Quarterly Report	true
Document Transition Report	false
Entity File Number	001-14761
Entity Tax Identification Number	13-4007862
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	One Corporate Center
Entity Address, City or Town	Rye
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10580
City Area Code	203
Local Phone Number	629-2726
Title of 12(b) Security	Class A Common Stock, $0.001 par value
Trading Symbol	GBL
Security Exchange Name	NYSE
Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		8,432,028
Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		19,024,117