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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, 2021
Dec. 31, 2020
Pension benefits    
Actuarial losses (gains)    
Balance, January 1 $ 57,231 $ 38,510
Additions to AOCI (59,754) 50,026
Amortization in current period (13,130) (5,430)
Amortization pursuant to plan settlements (210)  
Reclassification to regulatory accounts 31,670 (25,875)
Ending balance, December 31 15,807 57,231
Past service gains    
Beginning balance, January 1 (5,306) (6,180)
Additions to AOCI 237 (191)
Amortization in current period 1,626 1,609
Amortization pursuant to plan settlements 0  
Reclassification to regulatory accounts (752) (544)
Ending balance, December 31 (4,195) (5,306)
OPEB    
Actuarial losses (gains)    
Balance, January 1 (4,299) (9,146)
Additions to AOCI (24,126) 22,036
Amortization in current period (2,021) (509)
Amortization pursuant to plan settlements 0  
Reclassification to regulatory accounts 14,816 (16,680)
Ending balance, December 31 (15,630) (4,299)
Past service gains    
Beginning balance, January 1 0 0
Additions to AOCI 24 0
Amortization in current period 310 0
Amortization pursuant to plan settlements 0  
Reclassification to regulatory accounts 0 0
Ending balance, December 31 $ 334 $ 0