Retirement plans - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 62,400,000
Cash contributions	0	$ 0
Accrued liabilities | TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	6,000,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	900,000	$ 900,000
Additional cash payments	$ 5,000,000