Retirement plans - Expense recognized in other comprehensive income (Details) - USD ($)	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Employee Benefits [Abstract]
Actuarial gain (loss)	$ 7,499,000	$ (5,413,000)
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; Defined benefit plans [member]; Plan assets [member]; Net defined benefit liability (asset)] [Contrast: Decrease (increase) in net defined benefit liability (asset) resulting from gain (loss) on remeasurement in other comprehensive income]

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