Employee benefits - Change in accumulated other comprehensive income loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive loss at beginning of year	$ 225,356	$ 265,899
Recognized during the year:
Amortization of actuarial losses	(15,644)	(18,876)	$ (20,880)
Ocurring during the year:
Net actuarial (gains) / losses	33,722	(21,667)
Total (decrease) increase in AOCL	18,078	(40,543)
Accumulated other comprehensive loss at end of year	243,434	225,356	265,899
OPEB Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive loss at beginning of year	12,993	32,152
Recognized during the year:
Amortization of actuarial losses	0	(1,873)	(567)
Ocurring during the year:
Net actuarial (gains) / losses	(39,479)	(17,286)
Total (decrease) increase in AOCL	(39,479)	(19,159)
Accumulated other comprehensive loss at end of year	$ (26,486)	$ 12,993	$ 32,152