Schedule of Impact on Earnings Per Share Calculation (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
For earnings (loss) per share - basic:
Net loss attributable to Iconix Brand Group, Inc.		$ (7,336)	$ (109,522)
Accretion of redeemable non-controlling interest		(29)
Net loss attributable to Iconix Brand Group, Inc. after the effect of accretion of redeemable non-controlling interest for basic earnings (loss) per share		(7,365)	(109,522)
For earnings (loss) per share - diluted:
Net loss attributable to Iconix Brand Group, Inc.		(7,336)	(109,522)
Accretion of redeemable non-controlling interest		(29)
Net loss attributable to Iconix Brand Group, Inc. after the effect of potential conversion of 5.75% Convertible Notes for diluted earnings (loss) per share		$ (7,365)	$ (109,522)
Basic	$ 0.54	$ (0.60)	$ (10.37)
Diluted	$ 0.37	$ (0.60)	$ (10.37)
Weighted average number of common shares outstanding:
Basic		12,334	10,559
Diluted		12,334	10,559