Earnings per Share - Reconciliation of Net Income (Loss) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	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
Numerator:
Net income (loss) attributable to PQ Group Holdings Inc.	$ 65,011	$ (3,345)	$ (1,609)	$ (2,454)	$ 10,664	$ (10,017)	$ (77,262)	$ (3,131)	$ 57,603	$ (79,746)	$ 11,427
Denominator:
Weighted average shares outstanding – Basic (shares)	133,138,140	104,096,837	104,015,815	103,947,888	103,947,888	103,783,719	77,842,216	22,694,161	111,299,670	78,016,005	22,615,787
Weighted average shares outstanding – Diluted (shares)	133,895,646	104,096,837	104,015,815	103,947,888	103,947,888	103,783,719	77,842,216	22,694,161	111,669,037	78,016,005	22,615,787
Net income (loss) per share:
Basic income (loss) per share (usd per share)	$ 0.49	$ (0.03)	$ (0.02)	$ (0.02)	$ 0.10	$ (0.10)	$ (0.99)	$ (0.14)	$ 0.52	$ (1.02)	$ 0.51
Diluted income (loss) per share (usd per share)	$ 0.49	$ (0.03)	$ (0.02)	$ (0.02)	$ 0.10	$ (0.10)	$ (0.99)	$ (0.14)	$ 0.52	$ (1.02)	$ 0.51