Pension and postretirement benefits - Employer contribution (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2020
Pension Plans
Defined Benefit Plan Disclosure
Employer contribution	$ 57
Expected future employer contribution current year		$ 229
OPEB Plan
Defined Benefit Plan Disclosure
Employer contribution	$ 1,213
Expected future employer contribution current year		$ 6,516