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Retirement Plans - Schedule of Changes in Fair Value of Plan Assets (Level 3) (Details) - United Kingdom Subsidiary - UK Plan - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets at beginning of year | $ 257,236 | $ 263,555
Foreign currency exchange rate changes | 24,815 | (46,909)
Fair value of plan assets at end of year | 295,968 | 257,236
Level 3 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets at beginning of year | 0 |
Fair value of plan assets at end of year | 103,945 | 0
Corporate debt securities |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets at beginning of year | 103,912 |
Fair value of plan assets at end of year | 173,575 | 103,912
Corporate debt securities | Level 3 |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets at beginning of year | 0 |
Actual return on plan assets, relating to assets still held at reporting date | 1,858 |
Purchases, sales and settlements, net | 98,633 |
Foreign currency exchange rate changes | 3,454 |
Fair value of plan assets at end of year | $ 103,945 | $ 0

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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held. + 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 (d)(5)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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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 + Details
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- Definition Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan. + 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)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements. + 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 (d)(5)(iv)(02)(B) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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