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Retirement Plans - Schedule of Plan Assets Fair Value Hierarchy (Details) - United Kingdom Subsidiary - UK Plan - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 347,411	$ 307,001	$ 264,194
Corporate debt funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		65,486	64,314
Corporate debt funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Corporate debt funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		65,486	64,314
Corporate debt funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Government bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		57,133	49,164
Government bond funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Government bond funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		57,133	49,164
Government bond funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		44,132	32,356
Equity funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Equity funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		44,132	32,356
Equity funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		59,246	32,240
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		59,246	32,240
Cash | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Cash | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Total plan assets in fair value hierarchy
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		225,997	178,074
Total plan assets in fair value hierarchy | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		59,246	32,240
Total plan assets in fair value hierarchy | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		166,751	145,834
Total plan assets in fair value hierarchy | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Debt funds | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	96,196	103,188
Real estate funds | Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	$ 25,218	$ 25,739
[1] Certain investments measured using NAV as a practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the table are intended to permit reconciliation of the fair value hierarchy to the total fair value of 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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