Pension and other postretirement benefits - Summarized information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	$ 470,236	$ 416,470
Service cost	1,416	2,768	$ 1,500
Interest cost	12,827	16,000	14,816
Actuarial loss	36,726	57,525
Currency translation	2,273	229
Benefits paid	(21,092)	(20,350)
Medicare Part D reimbursement	0	0
Plan amendments	47	0
Administrative costs	(1,086)	(2,406)
Projected benefit obligation, end of year	501,347	470,236	416,470
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets, beginning of year	423,300	362,807
Actual return on plan assets	43,276	69,918
Contributions	12,490	12,695
Benefits paid	(21,092)	(20,350)
Administrative costs	(1,086)	(2,406)
Currency translation	738	636
Fair value of plan assets, end of year	457,626	423,300	362,807
Funded status, end of year	(43,721)	(46,936)
Other Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	40,042	42,115
Service cost	0	9	50
Interest cost	902	1,391	1,389
Actuarial loss	964	1,551
Currency translation	0	0
Benefits paid	(5,448)	(5,090)
Medicare Part D reimbursement	119	66
Plan amendments	(4,658)	0
Administrative costs	0	0
Projected benefit obligation, end of year	31,921	40,042	$ 42,115
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Administrative costs	0	0
Funded status, end of year	$ (31,921)	$ (40,042)