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Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Pension benefits    
Actuarial losses (gains)    
Balance, January 1 $ 15,807 $ 57,231
Additions to AOCI (47,473) (59,754)
Amortization in current period (3,429) (13,130)
Amortization due to plan settlements 15  
Amortization due to plan settlements   (210)
Reclassification to regulatory accounts 34,409 31,670
Ending balance, December 31 (671) 15,807
Past service losses (gains)    
Beginning balance, January 1 (4,195) (5,306)
Additions to AOCI 0 237
Amortization in current period 1,584 1,626
Amortization due to plan settlements 0  
Amortization due to plan settlements   0
Reclassification to regulatory accounts (752) (752)
Ending balance, December 31 (3,363) (4,195)
OPEB    
Actuarial losses (gains)    
Balance, January 1 (15,630) (4,299)
Additions to AOCI (41,527) (24,126)
Amortization in current period 56 (2,021)
Amortization due to plan settlements 0  
Amortization due to plan settlements   0
Reclassification to regulatory accounts 23,551 14,816
Ending balance, December 31 (33,550) (15,630)
Past service losses (gains)    
Beginning balance, January 1 310 0
Additions to AOCI (24) (24)
Amortization in current period (2,476) 334
Amortization due to plan settlements 0  
Amortization due to plan settlements   0
Reclassification to regulatory accounts 0 0
Ending balance, December 31 $ (2,190) $ 310