Derivative Contracts - Effects of Derivative Contracts in Condensed Consolidated Financial Statements (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	$ 4.2	$ 2.4	$ 4.2	$ 2.4	$ 4.0
Gross Amounts Offset in Consolidated Balance Sheet	(2.5)	(0.9)	(2.5)	(0.9)	(2.7)
Net Amounts of Assets Presented in Consolidated Balance Sheet	1.7	1.5	1.7	1.5	1.3
Consolidated balance sheet location:
Derivative contract assets	1.7	1.5	1.7	1.5	1.3
Gross Amounts of Recognized Liabilities	2.5	0.9	2.5	0.9	2.7
Gross Amounts Offset in Consolidated Balance Sheet	(2.5)	(0.9)	(2.5)	(0.9)	(2.7)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.0	0.0	0.0	0.0	0.0
Metal [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	1.4	0.9	1.4	0.9	0.5
Gross Amounts Offset in Consolidated Balance Sheet	(1.4)	(0.8)	(1.4)	(0.8)	(0.5)
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.0	0.1	0.0	0.1	0.0
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	1.7	0.8	1.7	0.8	1.4
Gross Amounts Offset in Consolidated Balance Sheet	(1.7)	(0.8)	(1.7)	(0.8)	(1.4)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.0	0.0	0.0	0.0	0.0
Natural Gas [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	0.0	0.1	0.0	0.1	0.0
Gross Amounts Offset in Consolidated Balance Sheet	0.0	0.0	0.0	0.0	0.0
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.0	0.1	0.0	0.1	0.0
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	0.4	0.0	0.4	0.0	0.7
Gross Amounts Offset in Consolidated Balance Sheet	(0.4)	0.0	(0.4)	0.0	(0.7)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.0	0.0	0.0	0.0	0.0
Electricity [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	0.0	0.4	0.0	0.4	0.2
Gross Amounts Offset in Consolidated Balance Sheet	0.0	(0.1)	0.0	(0.1)	(0.2)
Net Amounts of Assets Presented in Consolidated Balance Sheet	0.0	0.3	0.0	0.3	0.0
Consolidated balance sheet location:
Gross Amounts of Recognized Liabilities	0.4	0.1	0.4	0.1	0.6
Gross Amounts Offset in Consolidated Balance Sheet	(0.4)	(0.1)	(0.4)	(0.1)	(0.6)
Net Amounts of Liabilities Presented in Consolidated Balance Sheet	0.0	0.0	0.0	0.0	0.0
Collateral On Deposit [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets	2.8	1.0	2.8	1.0	3.3
Gross Amounts Offset in Consolidated Balance Sheet	(1.1)	0.0	(1.1)	0.0	(2.0)
Net Amounts of Assets Presented in Consolidated Balance Sheet	1.7	1.0	1.7	1.0	$ 1.3
Cost of Sales [Member]
Losses (gains) in cost of sales for:
Losses (gains) in cost of sales	0.7	(0.8)	0.3	(2.3)
Cost of Sales [Member] | Metal [Member]
Losses (gains) in cost of sales for:
Losses (gains) in cost of sales	0.4	(0.6)	(0.1)	(0.5)
Cost of Sales [Member] | Natural Gas [Member]
Losses (gains) in cost of sales for:
Losses (gains) in cost of sales	0.0	0.0	0.1	(0.2)
Cost of Sales [Member] | Electricity [Member]
Losses (gains) in cost of sales for:
Losses (gains) in cost of sales	$ 0.3	$ (0.2)	$ 0.3	$ (1.6)