Fair value measurement - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017
Fair Value Disclosures [Abstract]
Maximum amount of undiscounted payments under contingent consideration arrangements	401.8

For contingent consideration arrangements recognized in connection with a business combination, if the maximum (undiscounted) potential outcome is limitless, this element represents a statement of that fact.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Name:	us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesMaximumUnlimited
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration