Note Income taxes (Reconciliation of unrecognized tax benefits) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure
Beginning Balance	$ 7.3	$ 7.4
Addition for tax positions	0.2	0.2
Ending Balance	$ 7.5	$ 7.6

Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319

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