Retirement Plans - Information About Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
United States
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 4,459	$ 4,360	$ 12,468
United States | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets	399	817
United States | Total fixed income funds
Defined Benefit Plan Disclosure
Fair value of plan assets	1,440	1,336
United States | Corporate bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	1,440	1,336
United States | Total mutual funds and equities
Defined Benefit Plan Disclosure
Fair value of plan assets	2,620	2,207
United States | Mutual funds
Defined Benefit Plan Disclosure
Fair value of plan assets	713	694
United States | Equity securities
Defined Benefit Plan Disclosure
Fair value of plan assets	1,907	1,513
United Kingdom
Defined Benefit Plan Disclosure
Fair value of plan assets	7,975	7,290	$ 6,347
United Kingdom | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value of plan assets	348	516
United Kingdom | Bonds
Defined Benefit Plan Disclosure
Fair value of plan assets	3,410	3,735
United Kingdom | Other
Defined Benefit Plan Disclosure
Fair value of plan assets	1,116	949
United Kingdom | Equity securities
Defined Benefit Plan Disclosure
Fair value of plan assets	$ 3,101	$ 2,090