Retirement plans - Benefit Costs (Income) (Details) - TRP - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	$ 5,675	$ 6,150
Expected return on plan assets	(5,500)	(5,225)
Amortization of prior service cost (credit)	25	(125)
Amortization of actuarial loss	1,475	1,575
Expense for company-sponsored retirement plans	$ 1,675	$ 2,375