Retirement plans - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 78,800,000
Cash contributions	0	$ 0
TRP | Accrued Liabilities
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	5,600,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	2,800,000	$ 2,900,000
Additional cash payments	$ 2,100,000