Employee benefits - Change in accumulated other comprehensive income loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Amount recognized in AOCL at beginning of year, pre-tax	$ 200,094	$ 243,434
Recognized during the year:
Amortization of actuarial losses	(16,664)	(21,465)	$ (15,644)
Ocurring during the year:
Net actuarial (gains) / losses	(6,413)	(21,875)
Total (decrease) increase in AOCL	(23,077)	(43,340)
Accumulated other comprehensive loss at end of year	177,017	200,094	243,434
OPEB Plan
Defined Benefit Plan Disclosure [Line Items]
Amount recognized in AOCL at beginning of year, pre-tax	(25,454)	(26,486)
Recognized during the year:
Amortization of actuarial losses	2,193	2,212	0
Ocurring during the year:
Net actuarial (gains) / losses	(16,787)	(1,180)
Total (decrease) increase in AOCL	(14,594)	1,032
Accumulated other comprehensive loss at end of year	$ (40,048)	$ (25,454)	$ (26,486)