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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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