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Retirement Plans - Schedule of Plan Assets Fair Value Hierarchy (Details) - United Kingdom Subsidiary - UK Plan - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 295,968	$ 257,236	$ 263,555
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25,502	435
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	166,521	256,801
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103,945	0
Equity and equity like investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	41,684	115,416
Equity and equity like investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Equity and equity like investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	41,684	115,416
Equity and equity like investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	173,575	103,912
Corporate debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69,630	103,912
Corporate debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	103,945	0
Government bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55,207	37,473
Government bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Government bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55,207	37,473
Government bonds | 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	25,502	435
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25,502	435
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

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- Definition

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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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