Retirement plans - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 60,800,000
Cash contributions	0	$ 0
TRP | Accrued liabilities
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	6,000,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	1,900,000	$ 1,800,000
Additional cash payments	$ 3,600,000