| Quarterly Financial Summary (Unaudited) | 27. Quarterly Financial Summary (Unaudited): The following tables summarize the Company’s quarterly financial results during the years ended December 31, 2020 and 2019: 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  |  | 2020 |  
|  |  | First Quarter
 |  | Second Quarter
 |  | Third Quarter
 |  | Fourth Quarter
 |  
| Sales |  | $ | 295,813 |  |  | $ | 254,896 |  |  | $ | 275,149 |  |  | $ | 281,505 |  |  
| Gross profit |  | 72,295 |  |  | 63,662 |  |  | 70,172 |  |  | 67,227 |  |  
| Operating income |  | 29,214 |  |  | 20,175 |  |  | 33,310 |  |  | (245,623) |  |  
| Net income (loss) from continuing operations |  | 6,299 |  |  | 7,896 |  |  | (6,956) |  |  | (186,702) |  |  
| Net income (loss) from discontinued operations, net of tax |  | (5,790) |  |  | 8,351 |  |  | 11,392 |  |  | (116,194) |  |  
| Net (loss) income |  | 509 |  |  | 16,247 |  |  | 4,436 |  |  | (302,896) |  |  
| Less: Net (loss) income attributable to the noncontrolling interest - continuing operations |  | 234 |  |  | 250 |  |  | 201 |  |  | (3,883) |  |  
| Less: Net income (loss) attributable to the noncontrolling interest - discontinued operations |  | 51 |  |  | 71 |  |  | 97 |  |  | 46 |  |  
| Net income (loss) attributable to PQ Group Holdings Inc. |  | 224 |  |  | 15,926 |  |  | 4,138 |  |  | (299,059) |  |  
|  |  |  |  |  |  |  |  |  |  
| Earnings (loss) per common share - basic |  |  |  |  |  |  |  |  |  
| Continuing operations |  | $ | 0.04 |  |  | $ | 0.06 |  |  | $ | (0.05) |  |  | $ | (1.35) |  |  
| Discontinued operations |  | $ | (0.04) |  |  | $ | 0.06 |  |  | $ | 0.08 |  |  | $ | (0.86) |  |  
| Net earnings (loss) per share - basic |  | $ | — |  |  | $ | 0.12 |  |  | $ | 0.03 |  |  | $ | (2.21) |  |  
|  |  |  |  |  |  |  |  |  |  
| Earnings (loss) per common share - diluted: |  |  |  |  |  |  |  |  |  
| Continuing operations |  | $ | 0.04 |  |  | $ | 0.06 |  |  | $ | (0.05) |  |  | $ | (1.35) |  |  
| Discontinued operations |  | $ | (0.04) |  |  | $ | 0.06 |  |  | $ | 0.08 |  |  | $ | (0.86) |  |  
| Net earnings (loss) per share - diluted |  | $ | — |  |  | $ | 0.12 |  |  | $ | 0.03 |  |  | $ | (2.21) |  |  
|  |  |  |  |  |  |  |  |  |  
| Weighted average shares outstanding: |  |  |  |  |  |  |  |  |  
| Basic |  | 135,240,897 |  |  | 135,083,126 |  |  | 135,106,969 |  |  | 135,406,081 |  |  
| Diluted |  | 136,086,082 |  |  | 135,671,830 |  |  | 135,979,118 |  |  | 135,406,081 |  |  
|  |  |  |  |  |  |  |  |  | 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  |  | 2019 |  
|  |  | First Quarter
 |  | Second Quarter
 |  | Third Quarter
 |  | Fourth Quarter
 |  
| Sales |  | $ | 297,468 |  |  | $ | 311,641 |  |  | $ | 307,271 |  |  | $ | 283,535 |  |  
| Gross profit |  | 67,728 |  |  | 81,863 |  |  | 82,913 |  |  | 65,898 |  |  
| Operating income |  | 28,995 |  |  | 48,840 |  |  | 41,191 |  |  | 28,482 |  |  
| Net income from continuing operations |  | 6,055 |  |  | 19,404 |  |  | 11,888 |  |  | 28,406 |  |  
| Net income (loss) from discontinued operations, net of tax |  | (2,614) |  |  | 11,315 |  |  | 6,863 |  |  | (1,007) |  |  
| Net income |  | 3,441 |  |  | 30,719 |  |  | 18,751 |  |  | 27,399 |  |  
| Less: Net income attributable to the noncontrolling interest - continuing operations |  | 231 |  |  | 101 |  |  | 82 |  |  | 203 |  |  
| Less: Net income attributable to the noncontrolling interest - discontinued operations |  | 59 |  |  | 44 |  |  | 24 |  |  | 27 |  |  
| Net income attributable to PQ Group Holdings Inc. |  | 3,151 |  |  | 30,574 |  |  | 18,645 |  |  | 27,169 |  |  
|  |  |  |  |  |  |  |  |  |  
| Earnings (loss) per common share - basic: |  |  |  |  |  |  |  |  |  
| Continuing operations |  | $ | 0.04 |  |  | $ | 0.14 |  |  | $ | 0.09 |  |  | $ | 0.21 |  |  
| Discontinued operations |  | $ | (0.02) |  |  | $ | 0.08 |  |  | $ | 0.05 |  |  | $ | (0.01) |  |  
| Net earnings per share - basic |  | $ | 0.02 |  |  | $ | 0.23 |  |  | $ | 0.14 |  |  | $ | 0.20 |  |  
|  |  |  |  |  |  |  |  |  |  
| Earnings (loss) per common share - diluted: |  |  |  |  |  |  |  |  |  
| Continuing operations |  | $ | 0.04 |  |  | $ | 0.14 |  |  | $ | 0.09 |  |  | $ | 0.21 |  |  
| Discontinued operations |  | $ | (0.02) |  |  | $ | 0.08 |  |  | $ | 0.05 |  |  | $ | (0.01) |  |  
| Net earnings per share - diluted |  | $ | 0.02 |  |  | $ | 0.23 |  |  | $ | 0.14 |  |  | $ | 0.20 |  |  
|  |  |  |  |  |  |  |  |  |  
| Weighted average shares outstanding: |  |  |  |  |  |  |  |  |  
| Basic |  | 133,946,308 |  |  | 134,142,552 |  |  | 134,511,819 |  |  | 134,912,212 |  |  
| Diluted |  | 134,894,354 |  |  | 135,323,024 |  |  | 135,649,710 |  |  | 136,151,739 |  |  
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