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Employee Benefits - Plan Asset Fair Value Measurements (Details) - United Kingdom - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 27,297	$ 32,423	$ 24,876
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27,297	32,423
Fair Value, Inputs, Level 1 | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,029	4,086
Fair Value, Inputs, Level 1 | Fixed Income Securities | Legal and General Active Corporate Bond – Over 10 Year
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,688	7,188
Fair Value, Inputs, Level 1 | Fixed Income Securities | Legal and General Gilt Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7,136	7,611
Fair Value, Inputs, Level 1 | Fixed Income Securities | Legal and General Index Linked Gilt Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 12,444	$ 13,538

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+ References

No definition available.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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