Derivative Financial Instruments - Outstanding Copper Swap Contracts (Detail) - Copper Swap Contracts [Member] £ in Millions, $ in Millions	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 GBP (£)
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding | £		£ 49.1		£ 21.3
Net Fair Value - Asset (Liability) | $	$ 11.6		$ (10.5)
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding | £		42.6		17.3
Net Fair Value - Asset (Liability) | $	10.6		(9.8)
Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Copper Swap Contracts Units Outstanding | £		£ 6.5		£ 4.0
Net Fair Value - Asset (Liability) | $	$ 1.0		$ (0.7)