Business combinations - Schedule of Purchase Price Allocation (Detail) - USD ($) $ in Millions	Feb. 27, 2015	Sep. 30, 2015
Liabilities:
Goodwill		$ 85.0
Real Alloy Acquisition
Purchase consideration:
Final working capital adjustment	$ 2.4
Total purchase consideration	554.5
Assets:
Cash	10.2
Trade accounts receivable	150.1
Inventories	173.8
Property, plant and equipment	326.4
Deferred income taxes	6.1
Other	4.0
Identifiable intangible assets	21.1
Total assets	691.7
Liabilities:
Trade payables	112.4
Accrued liabilities	26.9
Accrued pension liabilities	46.0
Environmental liabilities	18.4
Other	13.7
Deferred income taxes	4.8
Total liabilities	222.2
Estimated fair value of net assets acquired	469.5
Total purchase consideration	554.5
Goodwill	85.0
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