Retirement plans - Pension Costs (Detail) - Retirement Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost—benefits earned during the period	$ 816	$ 920	$ 812
Interest cost on benefit obligation	26,111	23,800	23,558
Expected return on plan assets	(26,764)	(31,464)	(28,697)
Amortization of prior service costs	670	673	599
Amortization of actuarial loss	7,615	6,335	4,003
Total pension expense for company-sponsored retirement plans	$ 8,448	$ 264	$ 275