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, 2024	Dec. 31, 2023	Dec. 31, 2022
Change in fair value of plan assets:
Fair value of plan assets at beginning of year	$ 652,426
Fair value of plan assets at end of year	617,175	$ 652,426
Pension Plans
Change in benefit obligation:
Benefit obligation at beginning of year	635,794	628,175
Service cost	0	0	$ 0
Interest cost	30,234	31,548	19,199
Actuarial (gain) / loss	(31,747)	16,861
Benefits paid	(44,523)	(40,790)
Benefit obligation at end of year	589,758	635,794	628,175
Change in fair value of plan assets:
Fair value of plan assets at beginning of year	652,426	619,885
Actual return on plan assets	9,042	73,101
Employer contribution	230	230
Benefits paid	(44,523)	(40,790)
Fair value of plan assets at end of year	617,175	652,426	619,885
OPEB Plan
Change in benefit obligation:
Benefit obligation at beginning of year	117,045	118,336
Service cost	127	191	485
Interest cost	5,686	6,082	3,931
Actuarial (gain) / loss	(16,787)	(1,180)
Benefits paid	(6,899)	(6,384)
Benefit obligation at end of year	99,172	117,045	118,336
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	6,899	6,384
Benefits paid	(6,899)	(6,384)
Fair value of plan assets at end of year	$ 0	$ 0	$ 0