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Employee benefits - Reconciliation of net (liabilities) assets (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plans      
Reconciliation of net (liabilities) assets:      
Net (liabilities) assets at beginning of year $ (68,735) $ (49,449)  
Amount recognized in AOCL at beginning of year 304,330 290,327  
Amount prepaid (accued) at beginning of year 235,595 240,878  
Net periodic benefit (credit) cost (19,559) (5,513) $ (4,996)
Additional benefit cost 0 0  
Contributions 20,230 230  
Amount prepaid (accrued) at end of year 236,266 235,595 240,878
Accumulated other comprehensive loss (288,882) (304,330) (290,327)
Net (liabilities) assets at end of year (52,616) (68,735) (49,449)
OPEB Plan      
Reconciliation of net (liabilities) assets:      
Net (liabilities) assets at beginning of year (153,415) (170,720)  
Amount recognized in AOCL at beginning of year 5,720 24,079  
Amount prepaid (accued) at beginning of year (147,695) (146,641)  
Net periodic benefit (credit) cost (6,714) (4,402) (3,498)
Additional benefit cost 0 (1,790)  
Contributions 7,200 5,138  
Amount prepaid (accrued) at end of year (147,209) (147,695) (146,641)
Accumulated other comprehensive loss (21,472) (5,720) (24,079)
Net (liabilities) assets at end of year $ (168,681) $ (153,415) $ (170,720)