Schedule I - CONDENSED FINANCIAL STATEMENTS, CONDENSED STATEMENTS OF CASH FLOWS (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Nov. 18, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 15, 2016	Nov. 30, 2015	May 31, 2011
Operating activities [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders		$ 13,495	$ 16,600	$ 22,894	$ 24,820	$ 32,692	$ 30,861	$ 27,543	$ 26,025	$ 77,809	$ 117,121	$ 83,412
Loss from discontinued operations, net of taxes										0	0	3,887
Income from continuing operations										77,809	117,121	87,299
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization										595	625	618
Stock based compensation expense										8,669	3,959	9,868
Tax benefit from exercise of stock options										0	0	102
Donated securities										1,124	499	1,945
Gains on sales of available for sale securities										(62)	(4)	(6)
Accretion of zero coupon debentures										0	0	628
Loss on extinguishment of debt										3,300	0	1,067
(Increase) decrease in assets [Abstract]
Investments in trading securities										8	186	(240)
Receivable from affiliates										329	(927)	21,393
Receivable from brokers										(1,125)	638	592
Income tax receivable and deferred tax assets										(6,267)	(2,562)	(4,354)
Other assets										(73)	(69)	529
Increase (decrease) in liabilities [Abstract]
Payable to affiliates										(557)	(6,275)	7,333
Payable to brokers										(990)	54	1
Income taxes payable and deferred tax liabilities										4,473	2,768	(10,401)
Compensation payable										40,517	17,969	(6,369)
Accrued expenses and other liabilities										(714)	144	987
Total adjustments										48,882	(1,384)	29,831
Net cash provided by operating activities from continuing operations										126,691	115,737	117,130
Investing activities [Abstract]
Purchases of available for sale securities										(3,932)	(1,843)	(6,279)
Proceeds from sales of available for sale securities										4,169	408	81
Net cash provided by (used in) investing activities from continuing operations										237	(1,435)	(6,198)
Financing activities [Abstract]
Repurchase of Zero coupon subordinated debentures due December 31, 2015										0	0	(13,101)
Repurchase of 5.875% Senior note due June 1, 2021										0	0	(76,533)
Repayment of AC 4% PIK Note due November 30, 2020										(50,000)	(150,000)	0
Issuance of 4.5% Convertible note due August 15, 2021										0	109,826	0
Repayment of 4.5% Convertible note due August 15, 2021										(113,300)	0	0
Repayment of GGCP loan due December 28, 2016										0	(35,000)	0
Proceeds from GGCP loan due December 28, 2016										0	0	35,000
Proceeds from 1.6% AC Note de February 28, 2018										15,000	0	0
Margin loan borrowings										20,850	0	0
Margin loan repayments										(5,000)	0	0
Amortization of debt issuance costs										187	33	0
Proceeds from exercise of stock options										0	0	1,167
Dividends paid										(2,315)	(2,333)	(7,468)
Purchase of treasury stock										(14,324)	(10,773)	(27,249)
Net cash used in financing activities from continuing operations										(148,902)	(88,247)	(109,923)
Cash flows of discontinued operations [Abstract]
Net cash provided by operating activities										0	0	54,335
Net cash used in investing activities										0	0	(41,463)
Net cash used in financing activities										0	0	(12,871)
Net cash provided by discontinued operations										0	0	1
Net increase (decrease) in cash and cash equivalents										(21,991)	26,093	1,025
Cash and cash equivalents at beginning of period					39,812				13,719	39,812	13,719	12,694
Cash and cash equivalents at end of period		$ 17,821				39,812				17,821	39,812	13,719
Supplemental disclosures of cash flow information [Abstract]
Cash paid for interest										12,180	11,274	7,011
Cash paid for taxes										$ 62,259	75,238	59,657
5.875% Senior Notes [Member]
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Loss on extinguishment of debt	$ 800
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		5.875%								5.875%					5.875%
Maturity date										Jun. 01, 2021
AC 4% PIK Note [Member]
Financing activities [Abstract]
Repayment of AC 4% PIK Note due November 30, 2020										$ (50,000)
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		4.00%								4.00%				4.00%
Maturity date										Nov. 30, 2020
4.5% Convertible Notes [Member]
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Loss on extinguishment of debt		$ 3,300								$ 3,300
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		4.50%								4.50%			4.50%
Maturity date		Aug. 15, 2021								Aug. 15, 2021
Loan from GGCP [Member]
Supplemental disclosures of cash flow information [Abstract]
Maturity date										Dec. 28, 2016
AC 1.6% Note [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		1.60%								1.60%
Maturity date			Feb. 28, 2018							Feb. 28, 2018
Zero Coupon Subordinated Debentures [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		0.00%								0.00%
Maturity date										Dec. 31, 2015
Parent Company [Member]
Operating activities [Abstract]
Net income attributable to GAMCO Investors, Inc.'s shareholders										$ 77,809	117,121	83,412
Loss from discontinued operations, net of taxes										0	0	3,938
Income from continuing operations										77,809	117,121	87,350
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Depreciation and amortization										570	607	595
Stock based compensation expense										8,670	3,959	9,868
Tax benefit from exercise of stock options										0	0	102
Donated securities										1,124	499	1,945
Gains on sales of available for sale securities										(62)	(4)	(6)
Accretion of zero coupon debentures										0	0	628
Loss on extinguishment of debt										3,300	0	1,067
(Increase) decrease in assets [Abstract]
Investments in trading securities										(1)	(5)	0
Receivable from affiliates										(53,797)	(25,510)	(31,205)
Receivable from brokers										(1,192)	580	976
Income tax receivable and deferred tax assets										(77,832)	11,991	29,164
Other assets										136	(330)	(478)
Increase (decrease) in liabilities [Abstract]
Payable to affiliates										133,856	(30,567)	31,017
Payable to brokers										(990)	54	(539)
Income taxes payable and deferred tax liabilities										12,281	(1,137)	3,920
Compensation payable										45,779	16,351	(3,319)
Accrued expenses and other liabilities										(2,205)	(700)	(424)
Total adjustments										69,637	(24,212)	43,311
Net cash provided by operating activities from continuing operations										147,446	92,909	130,661
Investing activities [Abstract]
Purchases of available for sale securities										(3,932)	(1,843)	(6,279)
Proceeds from sales of available for sale securities										4,169	408	81
Net cash provided by (used in) investing activities from continuing operations										237	(1,435)	(6,198)
Financing activities [Abstract]
Repurchase of Zero coupon subordinated debentures due December 31, 2015										0	0	(13,101)
Repurchase of 5.875% Senior note due June 1, 2021										0	0	(76,533)
Repayment of AC 4% PIK Note due November 30, 2020										(50,000)	(150,000)	0
Issuance of 4.5% Convertible note due August 15, 2021										0	109,826	0
Repayment of 4.5% Convertible note due August 15, 2021										(113,300)	0	0
Repayment of GGCP loan due December 28, 2016										0	(35,000)	0
Proceeds from GGCP loan due December 28, 2016										0	0	35,000
Proceeds from 1.6% AC Note de February 28, 2018										15,000	0	0
Margin loan borrowings										20,850	0	0
Margin loan repayments										(5,000)	0	0
Amortization of debt issuance costs										187	33	0
Dividends paid by subsidiaries to GBL										0	0	164,000
Net transfer to/(from) AC										0	0	(196,297)
Proceeds from exercise of stock options										0	0	1,167
Dividends paid										(2,315)	(2,333)	(7,468)
Purchase of treasury stock										(14,324)	(10,773)	(27,249)
Net cash used in financing activities from continuing operations										(148,902)	(88,247)	(120,481)
Cash flows of discontinued operations [Abstract]
Net cash provided by operating activities										0	0	82,759
Net cash used in investing activities										0	0	(35,216)
Net cash used in financing activities										0	0	(47,543)
Net cash provided by discontinued operations										0	0	0
Net increase (decrease) in cash and cash equivalents										(1,219)	3,227	3,982
Cash and cash equivalents at beginning of period					$ 7,209				$ 3,982	7,209	3,982	0
Cash and cash equivalents at end of period		$ 5,990				$ 7,209				5,990	7,209	3,982
Supplemental disclosures of cash flow information [Abstract]
Cash paid for interest										10,891	10,425	6,282
Cash paid for taxes										$ 61,704	$ 74,457	$ 58,353
Parent Company [Member] | 5.875% Senior Notes [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		5.875%								5.875%
Maturity date										Jun. 01, 2021
Parent Company [Member] | AC 4% PIK Note [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		4.00%								4.00%
Maturity date										Nov. 30, 2020
Parent Company [Member] | 4.5% Convertible Notes [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		4.50%								4.50%
Maturity date										Aug. 15, 2021
Parent Company [Member] | Loan from GGCP [Member]
Supplemental disclosures of cash flow information [Abstract]
Maturity date										Dec. 28, 2016
Parent Company [Member] | AC 1.6% Note [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		1.60%								1.60%
Maturity date										Feb. 28, 2018
Parent Company [Member] | Zero Coupon Subordinated Debentures [Member]
Supplemental disclosures of cash flow information [Abstract]
Debt instrument, interest rate		0.00%								0.00%
Maturity date										Dec. 31, 2015