Business combinations - Schedule of Changes in Goodwill (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Preliminary purchase price allocation for the Real Alloy Acquisition reported as of March 31, 2015	$ 104.6
Balance at end of period	104.3
Real Alloy Acquisition
Business Acquisition [Line Items]
Preliminary purchase price allocation for the Real Alloy Acquisition reported as of March 31, 2015	102.3
Adjustments to preliminary purchase price allocation recorded in the measurement period	2.3
Balance at end of period	$ 104.6