Summary of Significant Accounting Policies - Schedule of Reconciliation Between Basic and Diluted Income Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net (loss) income attributable to common shareholders of Party City Holdco Inc.:	$ (96,417)	$ 239,707	$ (130,015)	$ (541,513)	$ (268,818)	$ (281,533)	$ 48,074	$ (30,218)	$ (528,238)	$ (532,495)	$ 123,259
Weighted-average number of common shares—Basic									100,804,944	93,295,692	96,133,144
Effect of dilutive restricted stock units:											9,661
Effect of dilutive stock options:											1,128,245
Weighted average shares — Diluted:									100,804,944	93,295,692	97,271,050
Net (loss) income per share attributable to common shareholders of Party City Holdco Inc. — Basic:	$ (0.88)	$ 2.25	$ (1.39)	$ (5.80)	$ (2.88)	$ (3.02)	$ 0.52	$ (0.32)	$ (5.24)	$ (5.71)	$ 1.28
Net (loss) income per share attributable to common shareholders of Party City Holdco Inc. — Diluted:	$ (0.88)	$ 2.24	$ (1.39)	$ (5.80)	$ (2.88)	$ (3.02)	$ 0.51	$ (0.32)	$ (5.24)	$ (5.71)	$ 1.27