Pension and other postretirement benefits - (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 416,470	$ 462,158
Service cost	2,768	1,500	$ 2,887
Interest cost	16,000	14,816	15,137
Actuarial (gain) loss	57,525	(38,446)
Currency translation	229	(1,780)
Benefits paid	(20,350)	(19,314)
Medicare Part D reimbursement	0	0
Plan amendments	0	157
Curtailments	0	(162)
Settlements	0	(1,420)
Administrative costs	(2,406)	(1,039)
Projected benefit obligation, end of year	470,236	416,470	462,158
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	362,807	386,307
Actual return on plan assets	69,918	(13,275)
Contributions	12,695	12,687
Benefits paid	(20,350)	(19,314)
Settlements	0	(1,420)
Administrative costs	(2,406)	(1,039)
Currency translation	636	(1,139)
Fair value of plan assets, end of year	423,300	362,807	386,307
Funded status, end of year	(46,936)	(53,663)
Other Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	42,115	48,903
Service cost	9	50	279
Interest cost	1,391	1,389	1,577
Actuarial (gain) loss	1,551	(6,058)
Currency translation	0	0
Benefits paid	(5,090)	(2,790)
Medicare Part D reimbursement	66	101
Plan amendments	0	0
Curtailments	0	520
Settlements	0	0
Administrative costs	0	0
Projected benefit obligation, end of year	40,042	42,115	$ 48,903
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Administrative costs	0	0
Funded status, end of year	$ (40,042)	$ (42,115)