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Employee benefits - Aggregate status of the plans and the amounts recognized in the consolidated financial statements (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Change in fair value of plan assets:      
Fair value of plan assets at beginning of year $ 619,885    
Fair value of plan assets at end of year 652,426 $ 619,885  
Pension Plans      
Change in benefit obligation:      
Benefit obligation at beginning of year 628,175 851,471  
Service cost 0 0 $ 0
Interest cost 31,548 19,199 15,993
Actuarial (gain) / loss 16,861 (194,473)  
Benefits paid (40,790) (48,022)  
Other Adjustments 0 0 0
Benefit obligation at end of year 635,794 628,175 851,471
Change in fair value of plan assets:      
Fair value of plan assets at beginning of year 619,885 860,484  
Actual return on plan assets 73,101 (192,807)  
Employer contribution 230 230  
Benefits paid (40,790) (48,022)  
Fair value of plan assets at end of year 652,426 619,885 860,484
OPEB Plan      
Change in benefit obligation:      
Benefit obligation at beginning of year 118,336 159,958  
Service cost 191 485 642
Interest cost 6,082 3,931 3,573
Actuarial (gain) / loss (1,180) (39,479)  
Benefits paid (6,384) (6,619)  
Other Adjustments 0 60 0
Benefit obligation at end of year 117,045 118,336 159,958
Change in fair value of plan assets:      
Fair value of plan assets at beginning of year 0 0  
Actual return on plan assets 0 0  
Employer contribution 6,384 6,619  
Benefits paid (6,384) (6,619)  
Fair value of plan assets at end of year $ 0 $ 0 $ 0