Note Rental expense and commitments (Parenthetical) (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Leases, Operating
Operating Leases Future Minimum Payments Due Future Minimum Sublease Rental	$ 0.3

Contractually required future rental payments receivable on noncancelable subleasing arrangements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41502-112717

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentals
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant