Derivative Contracts - Summary of Outstanding Commodity Derivative Contracts (Detail) $ in Millions	Jun. 30, 2015 USD ($) Contract	Dec. 31, 2014 USD ($) Contract	Jun. 30, 2014 USD ($) Contract
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 18.0	$ 21.9	$ 18.0
Number of Contracts | Contract	688	555	532
Notional Amount - Long [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 31.5	$ 29.9	$ 27.9
Notional Amount - Short [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	13.5	8.0	9.9
Metal [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 13.3	$ 13.4	$ 12.6
Number of Contracts | Contract	585	400	421
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 2.2	$ 2.6	$ 2.7
Number of Contracts | Contract	68	78	73
Electricity [Member]
Derivatives, Fair Value [Line Items]
Net Notional Amount	$ 2.5	$ 5.9	$ 2.7
Number of Contracts | Contract	35	77	38