Retirement plans - Pension Costs (Detail) - Retirement Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost-benefits earned during the period	$ 7	$ 8	$ 12
Interest cost on benefit obligation	19,487	23,066	21,337
Expected return on plan assets	(31,058)	(26,320)	(30,935)
Amortization of prior service cost	90	90	168
Amortization of actuarial loss	6,207	6,123	5,124
Pension payment timing related charge	0	686	7,498
(Gains from) expense for company-sponsored retirement plans	$ (5,267)	$ 3,653	$ 3,204