Business combinations - Summary of Preliminary Estimated Fair Value of Property Plant and Equipment (Detail) - Beck Aluminum International, LLC (“Beck Trading”) $ in Millions	Nov. 01, 2016 USD ($)
Business Acquisition [Line Items]
Preliminary estimated fair value	$ 6.8
Land and improvements
Business Acquisition [Line Items]
Preliminary estimated fair value	0.7
Buildings and improvements
Business Acquisition [Line Items]
Preliminary estimated fair value	2.6
Machinery, equipment, furniture and fixtures
Business Acquisition [Line Items]
Preliminary estimated fair value	$ 3.5

Fair value portion of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258

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