Employee benefit plans - Recognized Amounts in Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Total Recognized Amounts in the Consolidated Balance Sheets
Noncurrent liabilities	$ (142,900)	$ (120,084)
Retirement Plans
Total Recognized Amounts in the Consolidated Balance Sheets
Noncurrent assets	26,599	30,322
Current liabilities	(9,797)	(9,478)
Noncurrent liabilities	(134,671)	(120,562)
Net (liability)/asset	(117,869)	(99,718)
Retiree Health and Life Insurance Plans
Total Recognized Amounts in the Consolidated Balance Sheets
Noncurrent assets	0	2,919
Current liabilities	(1,801)	(1,049)
Noncurrent liabilities	(7,557)	(364)
Net (liability)/asset	$ (9,358)	$ 1,506