Business combinations - Schedule of Purchase Price Allocation (Detail) - USD ($) $ in Millions			3 Months Ended
	Sep. 03, 2015	Feb. 27, 2015	Jun. 30, 2015	Dec. 31, 2015
Liabilities:
Goodwill				$ 104.3
Real Alloy Acquisition
Purchase consideration:
Cash paid at closing		$ 501.2
Initial working capital adjustment			$ 31.3
Final working capital adjustment	$ 2.4
Total purchase consideration		554.5
Assets:
Cash		10.2
Trade accounts receivable		150.1
Inventories		157.7		6.4
Property, plant and equipment		311.7
Deferred income taxes		0.7
Prepaid expenses, supplies, and other current assets		19.9
Identifiable intangible assets		17.0
Total assets		667.3
Liabilities:
Trade payables		112.4
Accrued liabilities		26.9
Accrued pension benefits		46.0
Environmental liabilities		12.1
Noncurrent asset retirement obligations		5.9
Deferred income taxes		7.5
Other		6.6
Total liabilities		217.4
Estimated fair value of net assets acquired		449.9
Total purchase consideration		554.5
Estimated fair value of net assets acquired		449.9
Goodwill		104.6		$ 104.6
Real Alloy Acquisition | Redeemable Preferred Stock
Purchase consideration:
Fair value of Redeemable Preferred Stock issued		$ 19.6