Pension and other postretirement benefits - (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 430,574	$ 421,736
Service cost	2,887	2,615	$ 1,880
Interest cost	15,137	15,711	17,948
Actuarial loss	31,074	16,315
Currency translation	3,916	(4,300)
Benefits paid	(19,144)	(18,887)
Medicare Part D reimbursement	0	0
Plan amendments	0	0
Curtailments	0	(23)
Administrative costs	(2,286)	(2,593)
Projected benefit obligation, end of year	462,158	430,574	421,736
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	340,265	315,951
Actual return on plan assets	53,065	36,620
Contributions	12,670	12,752
Benefits paid	(19,144)	(18,887)
Administrative costs	(2,286)	(2,593)
Currency translation	1,737	(3,578)
Fair value of plan assets, end of year	386,307	340,265	315,951
Funded status, end of year	(75,851)	(90,309)
Other Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	47,487	48,616
Service cost	279	355	495
Interest cost	1,577	1,595	1,967
Actuarial loss	2,278	646
Currency translation	0	0
Benefits paid	(3,095)	(3,946)
Medicare Part D reimbursement	80	221
Plan amendments	297	0
Curtailments	0	0
Administrative costs	0	0
Projected benefit obligation, end of year	48,903	47,487	$ 48,616
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Benefits paid	(3,095)	(3,946)
Administrative costs	0	0
Funded status, end of year	$ (48,903)	$ (47,487)