Pension and other postretirement benefits - amounts recognized in consolidated balance sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Payroll and benefit-related liabilities	$ (107,415)	$ (82,679)
Pension and postretirement benefit liabilities	(121,410)	(133,062)
Pension
Defined Benefit Plan Disclosure [Line Items]
Other assets	1,596	106
Payroll and benefit-related liabilities	(1,767)	(1,640)
Pension and postretirement benefit liabilities	(75,680)	(88,775)
Accumulated other comprehensive loss	209,365	209,785
Amounts recognized in balance sheet	133,514	119,476
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Other assets	0	0
Payroll and benefit-related liabilities	(3,173)	(3,200)
Pension and postretirement benefit liabilities	(45,730)	(44,287)
Accumulated other comprehensive loss	6,715	4,415
Amounts recognized in balance sheet	$ (42,188)	$ (43,072)