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, 2020	Dec. 31, 2019	Dec. 31, 2018
Comprehensive Income (Loss) [Line Items]
Current year net actuarial gain (loss)	$ 22,574	$ (13,822)	$ (19,279)
Amortization of previously deferred prior service costs	6,209	5,764	5,141
Pension and other postretirement benefit items	28,783	(7,372)	(6,640)
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year net actuarial gain (loss)	23,597	(13,060)	(19,817)
Amortization of previously deferred actuarial loss	6,207	6,123	5,124
Amortization of previously deferred prior service costs	91	90	168
Pension payment timing related charges	0	686	7,498
Pension and other postretirement benefit items	$ 29,895	$ (6,161)	$ (7,027)