Annual report pursuant to Section 13 and 15(d)

Benefit Plans - Plan Assets (Details)

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Benefit Plans - Plan Assets (Details) - Defined Benefit Pension Plans - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure      
Fair value of plan assets $ 337,599 $ 287,542  
Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 152,722 195,949  
Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 179,477 86,572  
Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets 5,400 5,021  
Cash and cash equivalents      
Defined Benefit Plan Disclosure      
Fair value of plan assets 991 57,000  
Cash and cash equivalents | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 991 56,925  
Cash and cash equivalents | Level 1 | Eco Services Pension Equity and Hourly Pension Plans      
Defined Benefit Plan Disclosure      
Fair value of plan assets   55,905  
Cash and cash equivalents | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 75  
Cash and cash equivalents | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets $ 0 0  
Equity Securities | PQ Corporation Retirement Plan      
Defined Benefit Plan Disclosure      
Target asset plan allocation percentage 45.00%    
Equity Securities | Eco Services Pension Equity Plan      
Defined Benefit Plan Disclosure      
Target asset plan allocation percentage 40.00%    
Equity Securities | Eco Services Hourly Pension Plan      
Defined Benefit Plan Disclosure      
Target asset plan allocation percentage 60.00%    
U.S. investment funds      
Defined Benefit Plan Disclosure      
Fair value of plan assets $ 67,374 35,103  
U.S. investment funds | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 67,374 35,103  
U.S. investment funds | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 0  
U.S. investment funds | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 0  
International investment funds      
Defined Benefit Plan Disclosure      
Fair value of plan assets 65,136 44,508  
International investment funds | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 44,905 24,040  
International investment funds | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 20,231 20,468  
International investment funds | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets $ 0 0  
Fixed income securities | PQ Corporation Retirement Plan      
Defined Benefit Plan Disclosure      
Target asset plan allocation percentage 55.00%    
Fixed income securities | Eco Services Pension Equity Plan      
Defined Benefit Plan Disclosure      
Target asset plan allocation percentage 60.00%    
Fixed income securities | Eco Services Hourly Pension Plan      
Defined Benefit Plan Disclosure      
Target asset plan allocation percentage 40.00%    
Government securities      
Defined Benefit Plan Disclosure      
Fair value of plan assets $ 31,040 10,121  
Government securities | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 23,500 0  
Government securities | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 7,540 10,121  
Government securities | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 0  
Corporate bonds      
Defined Benefit Plan Disclosure      
Fair value of plan assets 14,900 77,229  
Corporate bonds | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 14,782 72,216  
Corporate bonds | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 118 5,013  
Corporate bonds | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 0  
Investment fund bonds      
Defined Benefit Plan Disclosure      
Fair value of plan assets 116,121 25,152  
Investment fund bonds | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 1,170 7,665  
Investment fund bonds | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 114,951 17,487  
Investment fund bonds | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 0  
Insurance contracts      
Defined Benefit Plan Disclosure      
Fair value of plan assets 42,037 38,429  
Insurance contracts | Level 1      
Defined Benefit Plan Disclosure      
Fair value of plan assets 0 0  
Insurance contracts | Level 2      
Defined Benefit Plan Disclosure      
Fair value of plan assets 36,637 33,408  
Insurance contracts | Level 3      
Defined Benefit Plan Disclosure      
Fair value of plan assets $ 5,400 $ 5,021 $ 4,150