Pension and other postretirement benefits - Amounts recognized in accumulated other comprehensive (income) loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan, Chance in Amounts Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, accumulated other comprehensive (income) loss, net of tax	$ 344,392
Impact of currency translation, accumulated other comprehensive (income) loss, net of tax	(59,758)	$ (4,195)	$ 83,889
Ending balance, accumulated other comprehensive (income) loss, net of tax	297,298	344,392
Pension
Defined Benefit Plan, Chance in Amounts Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, prior service cost (credit)	173	191
Net amortization and deferral, prior service cost	(15)	(18)
Actuarial changes in benefit obligation, prior service cost	0	0
Defined benefit plan, plan amendments	47
Impact of currency translation, prior service cost (credit)	0	0
Ending balance, prior service cost (credit)	205	173	191
Beginning balance, net (gain) or loss	213,816	205,719
Net amortization and deferral, net (gain) or loss	(7,432)	(6,995)
Actuarial changes in benefit obligation, net gain (loss)	25,100	15,033
Defined benefit plan, net (gain) or loss on plan amendments	0
Impact of currency translation, net (gain) or loss	851	59
Ending balance, net (gain) or loss	232,335	213,816	205,719
Beginning balance, deferred taxes	(76,270)	(74,429)
Net amortization and deferral, deferred taxes	1,738	1,631
Actuarial changes in benefit obligation, deferred taxes	(5,875)	(3,457)
Defined benefit plan, plan amendments on deferred taxes	(9)
Impact of currency translation, deferred taxes	(241)	(15)
Ending balance, deferred taxes	(80,657)	(76,270)	(74,429)
Beginning balance, accumulated other comprehensive (income) loss, net of tax	137,719	131,481
Net amortization and deferral, accumulated other comprehensive (income) loss, net of tax	(5,709)	(5,382)
Actuarial changes in benefit obligation, accumulated other comprehensive income (loss), net of tax	19,225	11,576
Plan amendments in benefit obligation, accumulated other comprehensive income loss, net of tax	38
Impact of currency translation, accumulated other comprehensive (income) loss, net of tax	610	44
Ending balance, accumulated other comprehensive (income) loss, net of tax	151,883	137,719	131,481
Other Benefits
Defined Benefit Plan, Chance in Amounts Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, prior service cost (credit)	7	71
Net amortization and deferral, prior service cost	(18)	(64)
Actuarial changes in benefit obligation, prior service cost	0	0
Defined benefit plan, plan amendments	(4,658)
Ending balance, prior service cost (credit)	(4,669)	7	71
Beginning balance, net (gain) or loss	1,909	293
Net amortization and deferral, net (gain) or loss	179	65
Actuarial changes in benefit obligation, net gain (loss)	964	1,551
Defined benefit plan, net (gain) or loss on plan amendments	0
Ending balance, net (gain) or loss	3,052	1,909	293
Beginning balance, deferred taxes	(825)	(465)
Net amortization and deferral, deferred taxes	(37)	0
Actuarial changes in benefit obligation, deferred taxes	(223)	(360)
Defined benefit plan, plan amendments on deferred taxes	1,076
Ending balance, deferred taxes	(9)	(825)	(465)
Beginning balance, accumulated other comprehensive (income) loss, net of tax	1,091	(101)
Net amortization and deferral, accumulated other comprehensive (income) loss, net of tax	124	1
Actuarial changes in benefit obligation, accumulated other comprehensive income (loss), net of tax	741	1,191
Plan amendments in benefit obligation, accumulated other comprehensive income loss, net of tax	(3,582)
Ending balance, accumulated other comprehensive (income) loss, net of tax	$ (1,626)	$ 1,091	$ (101)