Retirement plans - Expense recognized in other comprehensive income (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Employee Benefits [Abstract]
Actuarial loss	$ (5,413,000)	$ (4,479,000)
Minimum funding requirement	$ 0	$ 0

The amount of other comprehensive income, before tax, related to gains (losses) on remeasurements of defined benefit plans, which comprise actuarial gains and losses; the return on plan assets, excluding amounts included in net interest on the net defined benefit liability (asset); and any change in the effect of the asset ceiling, excluding amounts included in net interest on the net defined benefit liability (asset). [Refer: Other comprehensive income, before tax; Defined benefit plans [member]; Plan assets [member]; Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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-URIDate 2020-01-01

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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