Postretirement Benefit Plans - Weighted-Average Assumptions Used To Determine Benefit Obligations (Detail)	Dec. 31, 2017	Dec. 31, 2016
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	3.67%	4.17%
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	2.40%	2.60%

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant