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, 2019	Dec. 31, 2018	Dec. 31, 2017
Comprehensive Income (Loss) [Line Items]
Current year net actuarial loss	$ (13,822)	$ (19,279)	$ 20,373
Amortization of previously deferred prior service costs	5,764	5,141	8,837
Pension and other postretirement benefit items	(7,372)	(6,640)	29,210
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year net actuarial loss	(13,060)	(19,817)	16,272
Amortization of previously deferred actuarial loss	6,123	5,124	8,357
Amortization of previously deferred prior service costs	90	168	635
Pension payment timing related charges	686	7,498	0
Curtailment gain	0	0	4,716
Prior service cost recognized in curtailment	0	0	26
Pension and other postretirement benefit items	$ (6,161)	$ (7,027)	$ 30,006