Derivatives, Hedging Programs and Other Financial Instruments, Realized and Unrealized Gains and Losses Table (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Derivative Instruments Gain Loss [Line Items]
Cost of products sold, excluding depreciation and amortization and other items	$ 298.0	$ 298.6	$ 303.5	$ 315.1	$ 317.3	$ 323.3	$ 343.4	$ 316.7	$ 1,215.2	[1]	$ 1,300.7	[1]	$ 1,085.9	[1]
Cost of Sales
Derivative Instruments Gain Loss [Line Items]
Cost of products sold, excluding depreciation and amortization and other items													1,105.3
Cost of Sales | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									19.7		2.8		(0.9)
Cost of Sales | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									0.0		0.0		(19.8)
Cost of Sales | Aluminum | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									18.4		2.1		0.0
Cost of Sales | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									0.0		0.0		(20.4)
Cost of Sales | Natural Gas | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									0.2		(0.3)		0.0
Cost of Sales | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									0.0		0.0		0.7
Cost of Sales | Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									1.1		1.0		(0.9)
Cost of Sales | Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									0.0		0.0		(0.1)
Cost of Sales | Electricity | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains									$ 0.0		$ 0.0		0.0
Gain (Loss) on Derivative Instruments
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													(19.4)
Gain (Loss) on Derivative Instruments | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													0.0
Gain (Loss) on Derivative Instruments | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													(19.4)
Gain (Loss) on Derivative Instruments | Aluminum | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													0.0
Gain (Loss) on Derivative Instruments | Aluminum | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													(20.9)
Gain (Loss) on Derivative Instruments | Natural Gas | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													1.4
Gain (Loss) on Derivative Instruments | Alloy Metal Hedge | Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													0.0
Gain (Loss) on Derivative Instruments | Foreign Exchange Contract | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													0.0
Gain (Loss) on Derivative Instruments | Electricity | Not Designated as Hedging Instrument
Derivative Instruments Gain Loss [Line Items]
Realized (losses) gains													$ 0.1

[1] See Note 8 for discussion of our adoption of ASU 2017-12 (as defined in Note 1) in 2018 and the related reclassification of amounts in 2017 that were presented in the Statements of Consolidated Income within Unrealized (gain) loss on derivative instruments and are now presented within Cost of products sold, excluding depreciation and amortization and other items ("Cost of products sold").