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Fair value measurement - Fair Value of Pension Plan Assets by Level within Fair Value Hierarchy (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets $ 479,735    
Retirement Plans      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 479,735 $ 407,550 $ 439,149
Total 160,809 123,454  
Retirement Plans | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total 160,809 123,454  
Retirement Plans | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total 0 0  
Retirement Plans | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total 0 0  
Retirement Plans | Cash and other      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 1,395 958  
Retirement Plans | Cash and other | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 1,395 958  
Retirement Plans | Cash and other | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 0 0  
Retirement Plans | Cash and other | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 0 0  
Retirement Plans | Corporate stock      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 111,193 83,489  
Retirement Plans | Corporate stock | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 111,193 83,489  
Retirement Plans | Corporate stock | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 0 0  
Retirement Plans | Corporate stock | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 0 0  
Retirement Plans | Interest in registered investment companies      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 48,221 39,007  
Retirement Plans | Interest in registered investment companies | Level 1      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 48,221 39,007  
Retirement Plans | Interest in registered investment companies | Level 2      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 0 0  
Retirement Plans | Interest in registered investment companies | Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total fair value of plan assets 0 0  
Retirement Plans | Common collective trust - equities      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Pension plan investments valued using net asset value as a practical expedient: 111,385 104,993  
Retirement Plans | Common collective trust - fixed income      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Pension plan investments valued using net asset value as a practical expedient: 185,844 158,580  
Retirement Plans | Hedge funds      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Pension plan investments valued using net asset value as a practical expedient: 17,125 16,126  
Retirement Plans | Partnership/joint venture interests      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Pension plan investments valued using net asset value as a practical expedient: $ 4,572 $ 4,397