Employee benefit plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 3,200	$ 3,380
Expected return on assets	(4,028)	(4,298)
Amortization of net actuarial gain (loss)	1,037	773
Net periodic benefit cost (credit)	209	(145)
Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	126	138
Interest cost	459	489
Expected return on assets	(1,329)	(1,348)
Amortization of prior service credit	(330)	(330)
Amortization of net actuarial gain (loss)	(72)	(168)
Net periodic benefit credit, including amount capitalized	(1,146)	(1,219)
Less amount capitalized	22	24
Net periodic benefit cost (credit)	$ (1,168)	$ (1,243)