Employee benefit plans - Summary of Pension and Postretirement Life Insurance Plans (Details) - Pension Benefits - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in benefit obligation:
Benefit obligation, beginning	$ 16,338	$ 16,935
Interest cost	639	623	$ 650
Change in experience gain	96	135
Change in actuarial assumptions loss (gain)	1,221	(546)
Benefits paid	(803)	(809)
Benefit obligation, ending	17,491	16,338	16,935
Change in plan assets:
Fair value of plan assets, beginning	14,919	13,264
Actual return on plan assets	2,814	(236)
Employer contributions		2,700
Benefits paid	(803)	(809)
Fair value of plan assets, ending	16,930	14,919	$ 13,264
Funded status at end of year	$ (561)	$ (1,419)

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Defined Benefit Plan Change In Experience Gain Loss

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Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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