Business combinations - Schedule of Purchase Price Allocation (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Feb. 27, 2015
Liabilities:
Goodwill	$ 84.5
Real Alloy Acquisition
Purchase consideration:
Estimated final working capital adjustment	$ 2.4	$ 2.4
Total purchase consideration		554.5
Assets:
Cash		10.2
Trade accounts receivable		150.1
Inventories		173.9
Property, plant and equipment		326.4
Deferred income taxes		5.9
Other		4.0
Identifiable intangible assets		21.0
Total assets		691.5
Liabilities:
Trade payables		112.4
Accrued liabilities		26.7
Accrued pension liabilities		46.0
Environmental liabilities		18.4
Other		13.6
Deferred income taxes		4.4
Total liabilities		221.5
Estimated fair value of net assets acquired		470.0
Total purchase consideration		554.5
Goodwill		84.5
Real Alloy Acquisition | Consideration paid at closing
Purchase consideration:
Consideration paid		501.2
Real Alloy Acquisition | Consideration paid after closing
Purchase consideration:
Consideration paid		31.3
Real Alloy Acquisition | Redeemable Preferred Stock
Purchase consideration:
Redeemable Preferred Stock issued		$ 19.6