Pension and other postretirement benefits - amounts recognized in accumulated other comprehensive (income) loss (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Chance in Amounts Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, accumulated other comprehensive (income) loss, net of tax	$ 438,717
Impact of currency translation, accumulated other comprehensive (income) loss, net of tax	(173,074)	$ 69,162	$ 110,671
Ending balance, accumulated other comprehensive (income) loss, net of tax	265,091	438,717
Pension
Defined Benefit Plan, Chance in Amounts Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, prior service cost (credit)	79	113
Net amortization and deferral, prior service cost	(28)	(34)
Actuarial changes in benefit obligation, prior service cost	0	0
Curtailments, prior service cost (credit)	0	0
Impact of currency translation, prior service cost (credit)	0	0
Ending balance, prior service cost (credit)	51	79	113
Beginning balance, net (gain) or loss	209,706	213,188
Net amortization and deferral, net (gain) or loss	(6,706)	(6,533)
Actuarial changes in benefit obligation, net gain (loss)	4,818	4,481
Curtailments, Net (Gain) or Loss	0	(23)
Impact of currency translation, net (gain) or loss	1,496	(1,407)
Ending balance, net (gain) or loss	209,314	209,706	213,188
Beginning balance, deferred taxes	(76,140)	(77,255)
Net amortization and deferral, deferred taxes	2,395	2,339
Actuarial changes in benefit obligation, deferred taxes	(1,119)	(1,603)
Curtailments, deferred taxes	0	6
Impact of currency translation, deferred taxes	(413)	373
Ending balance, deferred taxes	(75,277)	(76,140)	(77,255)
Beginning balance, accumulated other comprehensive (income) loss, net of tax	133,645	136,046
Net amortization and deferral, accumulated other comprehensive (income) loss, net of tax	(4,339)	(4,228)
Actuarial changes in benefit obligation, accumulated other comprehensive income (loss), net of tax	3,699	2,878
Curtailments, accumulated other comprehensive income (loss), net of tax	0	(17)
Impact of currency translation, accumulated other comprehensive (income) loss, net of tax	1,083	(1,034)
Ending balance, accumulated other comprehensive (income) loss, net of tax	134,088	133,645	136,046
Other Benefits
Defined Benefit Plan, Chance in Amounts Recognized in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance, prior service cost (credit)	85	107
Net amortization and deferral, prior service cost	(77)	(22)
Actuarial changes in benefit obligation, prior service cost	0	0
Ending balance, prior service cost (credit)	305	85	107
Beginning balance, net (gain) or loss	4,330	4,116
Net amortization and deferral, net (gain) or loss	(198)	(432)
Actuarial changes in benefit obligation, net gain (loss)	2,278	646
Ending balance, net (gain) or loss	6,410	4,330	4,116
Beginning balance, deferred taxes	(1,464)	(1,382)
Net amortization and deferral, deferred taxes	101	170
Actuarial changes in benefit obligation, deferred taxes	(558)	(252)
Ending balance, deferred taxes	(1,995)	(1,464)	(1,382)
Beginning balance, accumulated other comprehensive (income) loss, net of tax	2,951	2,841
Actuarial changes in benefit obligation, accumulated other comprehensive income (loss), net of tax	1,720	394
Defined benefit plan, plan amendments and prior service costs	297
Defined benefit plan, net (gain) or loss on plan amendments	0
Defined benefit plan, plan amendments on deferred taxes	(74)
Plan amendments in benefit obligation, accumulated other comprehensive income loss, net of tax	223
Ending balance, accumulated other comprehensive (income) loss, net of tax	$ 4,720	$ 2,951	$ 2,841