Pension and postretirement benefits - Contributions to the benefit plans (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	$ 57
Expected future employer contribution current year	229
OPEB Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	1,567
Expected future employer contribution current year	$ 6,333