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Quarterly Financial Information (Unaudited) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Jan. 05, 2018
Dec. 27, 2017
Aug. 07, 2017
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Aug. 15, 2016
Quarterly Financial Information (Unaudited) [Abstract]                              
Revenues       $ 80,977 $ 85,788 $ 87,193 $ 87,497 $ 98,666 $ 88,341 $ 87,600 $ 85,917 $ 341,455 $ 360,524 $ 353,000  
Operating income       52,458 51,700 41,373 41,256 39,524 23,393 39,660 42,443 186,787 145,020 191,796  
Net income attributable to GAMCO Investors, Inc.'s shareholders       $ 23,337 $ 35,016 $ 31,582 $ 27,261 $ 13,495 $ 16,600 $ 22,894 $ 24,820 $ 117,196 $ 77,809 $ 117,121  
Net income attributable to GAMCO Investors, Inc.'s shareholders per share [Abstract]                              
Basic (in dollars per share)       $ 0.82 $ 1.22 $ 1.10 $ 0.94 $ 0.46 $ 0.57 $ 0.79 $ 0.86 $ 4.08 $ 2.68 $ 4.01  
Diluted (in dollars per share)       $ 0.81 $ 1.22 $ 1.10 $ 0.94 $ 0.46 $ 0.55 $ 0.76 $ 0.82 $ 4.07 $ 2.60 $ 3.92  
Debt Instrument [Line Items]                              
Loss on extinguishment of debt                       $ 0 $ (3,300) $ 0  
Write down of net deferred tax assets               $ 8,200         8,200    
4.5% Convertible Notes [Member]                              
Debt Instrument [Line Items]                              
Loss on extinguishment of debt               $ (3,300)         $ (3,300)    
Debt instrument, repurchased face amount       $ 110,000               $ 110,000      
Debt instrument, interest rate       4.50%               4.50%     4.50%
Debt instrument, maturity date               Aug. 15, 2021       Aug. 15, 2021      
Restricted Stock Awards [Member]                              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                              
Number of shares with accelerated vesting (in shares) 19,400 144,650 201,120         144,650       19,400 345,770    
Stock compensation expense resulting from accelerated lapsing of restrictions on RSAs $ 200 $ 1,300 $ 1,800         $ 1,300       $ 200 $ 6,800    
Stock compensation expense resulting from accelerated lapsing of restrictions on RSAs per fully diluted share (in dollars per share)               $ 0.02