Retirement plans - Benefit Costs (Income) (Details) - TRP - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	$ 5,697	$ 6,116	$ 11,372	$ 12,266
Expected return on plan assets	(5,533)	(5,245)	(11,033)	(10,470)
Amortization of prior service cost (credit)	20	(107)	45	(232)
Amortization of actuarial loss	1,475	1,434	2,950	3,009
Expense for company-sponsored retirement plans	$ 1,659	$ 2,198	$ 3,334	$ 4,573