Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2021	Jul. 31, 2021
Entity Information [Line Items]
Entity Registrant Name	GAMCO INVESTORS, INC. ET AL
Entity Central Index Key	0001060349
Current Fiscal Year End Date	--12-31
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Document Transition Report	false
Entity File Number	001-14761
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	13-4007862
Entity Address, Address Line One	191 Mason Street
Entity Address, City or Town	Greenwich
Entity Address, State or Province	CT
Entity Address, Postal Zip Code	06830
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
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		8,165,225
Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		19,024,117