Postretirement Benefit Plans - Defined Benefit Pension Plans - Additional Information (Detail)	6 Months Ended
Jun. 30, 2018 USD ($)
Defined Contribution Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Balance of trust assets	$ 1,670,000
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected payment related to non-qualified plan	312,000
Payments related to non-qualified plans	200,000
U.K Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected contribution to qualified plan	415,000
Employer contributions	$ 251,000

Balance of the trust assets.

No definition available.

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- Definition

Expected current year payments to be made out of an entity's cash to participants of an unfunded nonqualified pension plan.

+ References

No definition available.

+ Details

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- Definition

Current year payments made out of an entity's cash to participants of an unfunded nonqualified pension plan.

+ References

No definition available.

+ Details

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- Definition

Amount of contribution received by defined benefit plan from employer which increases plan assets.

+ 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)(ii)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 4: 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)(01)
-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

Amount of benefits for defined benefit plan expected to be paid in remainder of fiscal year.

+ References

No definition available.

+ Details

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- Details

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- Details

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