Derivatives, Hedging Programs and Other Financial Instruments Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ 21.2	$ 5.8
Derivative Liability	(1.8)	(1.8)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	34.4	55.2
Derivative Asset	21.2	5.8
Derivative Liability	(1.8)	(1.8)
Other Assets, Fair Value Disclosure	229.9	286.2
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	23.5	37.9
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Other Assets, Fair Value Disclosure	23.5	37.9
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	10.9	17.3
Derivative Asset	21.2	5.8
Derivative Liability	(1.8)	(1.8)
Other Assets, Fair Value Disclosure	206.4	248.3
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0.0	0.0
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Other Assets, Fair Value Disclosure	0.0	0.0
Not Designated as Hedging Instrument | Put Option [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.1
Not Designated as Hedging Instrument | Purchase | Midwest premium swap contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	1.8	0.9
Derivative Liability	(0.9)	(0.2)
Not Designated as Hedging Instrument | Purchase | Midwest premium swap contracts [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Midwest premium swap contracts [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	1.8	0.9
Derivative Liability	(0.9)	(0.2)
Not Designated as Hedging Instrument | Purchase | Midwest premium swap contracts [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Natural Gas | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.7	1.6
Derivative Liability	(0.7)	(0.4)
Not Designated as Hedging Instrument | Purchase | Natural Gas | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Natural Gas | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.7	1.6
Derivative Liability	(0.7)	(0.4)
Not Designated as Hedging Instrument | Purchase | Natural Gas | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Aluminum Member | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	18.4	3.3
Derivative Liability	(0.1)	(1.1)
Not Designated as Hedging Instrument | Purchase | Aluminum Member | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Aluminum Member | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	18.4	3.3
Derivative Liability	(0.1)	(1.1)
Not Designated as Hedging Instrument | Purchase | Aluminum Member | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0	0.0
Derivative Liability	0.0	0.0
Not Designated as Hedging Instrument | Purchase | Put Option [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Not Designated as Hedging Instrument | Purchase | Put Option [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.1
Not Designated as Hedging Instrument | Purchase | Put Option [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Designated as Hedging Instrument | Purchase | Alloy Metal Hedge [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.2
Derivative Liability	(0.1)	(0.1)
Designated as Hedging Instrument | Purchase | Alloy Metal Hedge [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Derivative Liability	0.0	0.0
Designated as Hedging Instrument | Purchase | Alloy Metal Hedge [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.2
Derivative Liability	(0.1)	(0.1)
Designated as Hedging Instrument | Purchase | Alloy Metal Hedge [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	0.0
Derivative Liability	0.0	0.0
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	195.5	231.0
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	195.5	231.0
Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.0	0.0
Other Current Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	17.2	5.0
Other Assets [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	4.0	0.8
Other Current Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1.1	0.8
Other Noncurrent Liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	$ 0.7	$ 1.0