Subsequent Events - Additional Information (Detail) - 1 months ended Jan. 31, 2019 £ in Millions, $ in Millions	USD ($)	GBP (£)
Subsequent Event [Member]
Subsequent Event [Line Items]
Payment to acquire building	$ 20.7	£ 16.0

The cash outflow for acquisition of buildings (properties) whether for investment or use.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

Name:	us-gaap_PaymentsToAcquireBuildings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration