Retirement plans - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 82,400,000
Cash contributions	0	$ 0
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	1,800,000	$ 3,200,000
Additional cash payments	5,100,000
Accrued liabilities | TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 7,800,000