Employee benefits - Funded Status of the plan (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Funded status of the plan:
Fair value of plan assets at end of year	$ 652,426	$ 619,885
Pension Plans
Funded status of the plan:
Benefit obligation at end of year	(635,794)	(628,175)	$ (851,471)
Fair value of plan assets at end of year	652,426	619,885	860,484
Funded status at year end	16,632	(8,290)	9,013
OPEB Plan
Funded status of the plan:
Benefit obligation at end of year	(117,045)	(118,336)	(159,958)
Fair value of plan assets at end of year	0	0	0
Funded status at year end	$ (117,045)	$ (118,336)	$ (159,958)