Earnings Per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Basic:
Net income attributable to GAMCO Investors, Inc.'s shareholders	$ 75,539	$ 69,682	$ 68,792
Weighted average shares outstanding (in shares)	26,283	26,636	26,959
Basic net income attributable to GAMCO Investors, Inc.'s shareholders per share (in dollars per share)	$ 2.87	$ 2.62	$ 2.55
Diluted:
Net income attributable to GAMCO Investors, Inc.'s shareholders	75,539	69,682	68,792
Add interest expense on convertible notes, net of management fee and taxes	0	0	2,521
Total	$ 75,539	$ 69,682	$ 71,313
Weighted average shares outstanding (in shares)	26,283	26,636	26,959
Dilutive stock options and restricted stock awards (in shares)	153	88	249
Assumed conversion of convertible notes (in shares)	0	0	1,140
Total (in shares)	26,436	26,724	28,348
Diluted net income attributable to GAMCO Investors, Inc.'s shareholders per share (in dollars per share)	$ 2.86	$ 2.61	$ 2.52