Pension and other postretirement benefits - Amounts recognized in consolidated balance sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Payroll and benefit-related liabilities	$ (122,366)	$ (115,981)
Pension and postretirement benefit liabilities	(74,499)	(82,719)
Pension
Defined Benefit Plan Disclosure [Line Items]
Other assets	3,703	2,449
Payroll and benefit-related liabilities	(1,721)	(1,617)
Pension and postretirement benefit liabilities	(45,703)	(47,768)
Accumulated other comprehensive loss (gain)	232,540	213,989
Amounts recognized in balance sheet	188,819	167,053
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Other assets	0	0
Payroll and benefit-related liabilities	(3,125)	(5,091)
Pension and postretirement benefit liabilities	(28,796)	(34,951)
Accumulated other comprehensive loss (gain)	(1,617)	1,916
Amounts recognized in balance sheet	$ (33,538)	$ (38,126)