Pension and other postretirement benefits - amounts recognized in consolidated balance sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Payroll and benefit-related liabilities	$ (104,670)	$ (107,415)
Pension and postretirement benefit liabilities	(92,914)	(121,410)
Pension
Defined Benefit Plan Disclosure [Line Items]
Other assets	2,837	1,596
Payroll and benefit-related liabilities	(1,729)	(1,767)
Pension and postretirement benefit liabilities	(54,771)	(75,680)
Accumulated other comprehensive loss	205,910	209,365
Amounts recognized in balance sheet	152,247	133,514
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Other assets	0	0
Payroll and benefit-related liabilities	(3,972)	(3,173)
Pension and postretirement benefit liabilities	(38,143)	(45,730)
Accumulated other comprehensive loss	364	6,715
Amounts recognized in balance sheet	$ (41,751)	$ (42,188)