Derivatives and Fair Value Measurements - Assets and Liabilities Measured and Recorded at Fair Value (Detail) - USD ($) $ in Millions	Mar. 31, 2020		Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Asset Derivatives, Fair Value	$ 1.2		$ 5.1
Liability Derivatives, Fair Value	13.1	[1]	9.6	[2]
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Asset Derivatives, Fair Value	1.2		5.1
Liability Derivatives, Fair Value	13.1	[3]	9.6	[4]
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivatives Not Designated as Hedging Instruments under ASC 815 [Member] | Metal Commodity Contracts [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Asset Derivatives, Fair Value	1.0		5.0
Liability Derivatives, Fair Value	8.2	[3]	9.4	[4]
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivatives Not Designated as Hedging Instruments under ASC 815 [Member] | Foreign Exchange Contracts [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Asset Derivatives, Fair Value	0.2		0.1
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Derivatives designated as hedging instruments under ASC 815 [Member] | Interest Rate Swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Liability Derivatives, Fair Value	$ 4.9		$ 0.2

[1] The offsetting cash collateral balance of $0.5 million held by the derivative counterparty brings the net metal commodity contract liability to $7.7 million and the net total derivative liability balance to $12.6 million as of March 31, 2020.

[2] The offsetting cash collateral balance of $2.7 million held by the derivative counterparty brought the net metal commodity contract liability to $6.7 million and the net total derivative liability balance to $6.9 million as of December 31, 2019.

[3] The offsetting cash collateral balance of $0.5 million held by the derivative counterparty brings the net metal commodity contract liability to $7.7 million and the net total derivative liability balance to $12.6 million.

[4] The offsetting cash collateral balance of $2.7 million held by the derivative counterparty brought the net metal commodity contract liability to $6.7 million and the net total derivative liability balance to $6.9 million