Pension and other postretirement benefits - (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Settlements	$ (1,420)	$ 0
Pension
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	462,158	430,574
Service cost	1,500	2,887	$ 2,615
Interest cost	14,816	15,137	15,711
Actuarial (gain) loss	(38,446)	31,074
Currency translation	(1,780)	3,916
Benefits paid	(19,314)	(19,144)
Medicare Part D reimbursement	0	0
Plan amendments	157	0
Curtailments	(162)	0
Settlements	(1,420)	0
Administrative costs	(1,039)	(2,286)
Projected benefit obligation, end of year	416,470	462,158	430,574
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	386,307	340,265
Actual return on plan assets	(13,275)	53,065
Contributions	12,687	12,670
Benefits paid	(19,314)	(19,144)
Administrative costs	(1,039)	(2,286)
Currency translation	(1,139)	1,737
Fair value of plan assets, end of year	362,807	386,307	340,265
Funded status, end of year	(53,663)	(75,851)
Other Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	48,903	47,487
Service cost	50	279	355
Interest cost	1,389	1,577	1,595
Actuarial (gain) loss	(6,058)	2,278
Currency translation	0	0
Benefits paid	(2,790)	(3,095)
Medicare Part D reimbursement	101	80
Plan amendments	0	297
Curtailments	520	0
Settlements	0	0
Administrative costs	0	0
Projected benefit obligation, end of year	42,115	48,903	$ 47,487
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Administrative costs	0	0
Funded status, end of year	$ (42,115)	$ (48,903)