Employee benefit plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 3,200	$ 3,380	$ 6,400	$ 7,169
Expected return on assets	(4,028)	(4,299)	(8,056)	(9,048)
Amortization of net actuarial gain (loss)	1,037	773	2,074	1,674
Net periodic benefit cost (credit)	209	(146)	418	(205)
Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	126	136	252	274
Interest cost	459	489	918	978
Expected return on assets	(1,329)	(1,334)	(2,658)	(2,668)
Amortization of prior service credit	(330)	(329)	(660)	(659)
Amortization of net actuarial gain (loss)	(72)	(70)	(144)	(126)
Net periodic benefit credit, including amount capitalized	(1,146)	(1,108)	(2,292)	(2,201)
Less amount capitalized	47	53	69	77
Net periodic benefit cost (credit)	$ (1,193)	$ (1,161)	$ (2,361)	$ (2,278)