Pension and other postretirement benefits - amounts recognized in consolidated balance sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Payroll and benefit-related liabilities	$ (84,658)	$ (85,442)
Pension and postretirement benefit liabilities	(149,441)	(167,241)
Pension
Defined Benefit Plan Disclosure [Line Items]
Payroll and benefit-related liabilities	(1,653)	(1,779)
Pension and postretirement benefit liabilities	(104,132)	(117,355)
Accumulated other comprehensive loss	213,301	213,117
Amounts recognized in balance sheet	107,516	93,983
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Payroll and benefit-related liabilities	(3,307)	(3,268)
Pension and postretirement benefit liabilities	(45,309)	(49,886)
Accumulated other comprehensive loss	4,223	8,353
Amounts recognized in balance sheet	$ (44,393)	$ (44,801)