Earnings per Share - Reconciliation of Net Income (Details) - 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
Numerator:
Net income attributable to PQ Group Holdings Inc.	$ (299,059)	$ 4,138	$ 15,926	$ 224	$ 27,169	$ 18,645	$ 30,574	$ 3,151	$ (278,771)	$ 79,539	$ 58,300
Denominator:
Weighted average shares outstanding – Basic (shares)	135,406,081	135,106,969	135,083,126	135,240,897	134,912,212	134,511,819	134,142,552	133,946,308	135,528,977	134,389,667	133,380,567
Weighted average shares outstanding – Diluted (shares)	135,406,081	135,979,118	135,671,830	136,086,082	136,151,739	135,649,710	135,323,024	134,894,354	135,528,977	135,548,694	134,684,931
Net (loss) income per share:
Basic (loss) income per share	$ (2.21)	$ 0.03	$ 0.12	$ 0	$ 0.20	$ 0.14	$ 0.23	$ 0.02	$ (2.06)	$ 0.59	$ 0.44
Diluted (loss) income per share	$ (2.21)	$ 0.03	$ 0.12	$ 0	$ 0.20	$ 0.14	$ 0.23	$ 0.02	$ (2.06)	$ 0.59	$ 0.43