Retirement plans - Narrative (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 121,100,000
Cash contributions	0	$ 2,400,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	4,100,000	$ 6,000,000.0
Additional cash payments	1,800,000
Accrued liabilities | TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 6,800,000