Retirement plans - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 124,900,000
Cash contributions	0	$ 900,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Cash contributions	3,200,000	$ 5,000,000.0
Additional cash payments	2,700,000
Accrued liabilities | TRP
Defined Benefit Plan Disclosure [Line Items]
Total net pension obligations	$ 6,800,000