Derivatives and Fair Value Measurements - Assets and Liabilities Measured and Recorded at Fair Value (Parenthetical) (Detail) - Fair Value, Measurements, Recurring [Member] - Level 2 [Member] - Derivative Counterparty [Member] - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative liability balance	$ 12.6	$ 6.9
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]
Derivative liabilities, offsetting cash collateral	0.5	2.7
Derivative liability balance	$ 7.7	$ 6.7