Retirement plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Comprehensive Income (Loss) [Line Items]
Current year net actuarial gain (loss)	$ 4,463	$ 22,574	$ (13,822)
Amortization of previously deferred prior service costs	5,217	6,209	5,764
Pension and other postretirement benefit items	10,626	28,783	(7,372)
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year net actuarial gain (loss)	4,463	23,597	(13,060)
Amortization of actuarial loss	4,952	6,207	6,123
Amortization of previously deferred prior service costs	90	91	90
Pension payment timing related charges	946	0	686
Pension and other postretirement benefit items	$ 10,451	$ 29,895	$ (6,161)