Pension and other postretirement benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 29, 2015
Compensation and Retirement Disclosure [Abstract]
Expected employer contribution for year 2014	$ 2.9
Employer contribution to pension plan	$ 1.2

The increase in the fair value of plan assets from contributions made by the employer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=58740216&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration