Retirement plans - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 128,700,000
Cash contributions	0
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	2,300,000	$ 900,000
Additional cash payments	4,400,000
Accrued liabilities | TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 6,800,000