Retirement Plans - Accumulated Benefit Obligations in Excess of Plan Asset (Details) - Pension Plan - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
United States
Defined Benefit Plan Disclosure
Projected benefit obligation	$ 7,809	$ 16,717	$ 18,783
Accumulated benefit obligation	7,809	16,717
Fair value of plan asset	4,360	12,468	14,892
United Kingdom
Defined Benefit Plan Disclosure
Projected benefit obligation	9,101	7,750	8,335
Accumulated benefit obligation	9,101	7,750
Fair value of plan asset	$ 7,290	$ 6,347	$ 6,904

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

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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 accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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