Fair Value Measurements, Level 3 Fair Value Input Reconciliation Table (Details) (Derivative, Midwest premium swap contracts, USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2013
Derivative \| Midwest premium swap contracts
Reconciliation of activity for financial instruments classified as Level 3:
Balance at December 31, 2012	$ 0.4
Total realized/unrealized gains included in:
Cost of goods sold excluding depreciation and amortization and other items and Unrealized losses (gains) on derivative instruments	(0.2)
Transactions involving Level 3 derivative contracts:
Purchases	(0.8)
Sales	0
Issuances	0
Settlements	(0.2)
Transactions involving Level 3 derivatives — net	(1.0)
Transfers in and (or) out of Level 3 valuation hierarchy	0
Balance at September 30, 2012	(0.8)
Total gains included in Unrealized losses (gains) on derivative instruments, attributable to the change in unrealized gains/losses relating to derivative contracts held at September 30, 2013:	$ (0.1)

Fair Value Assets Measured on Recurring Basis Change in Unrealized Gain (Loss) Included in Earnings.

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Amount of gain (loss) recognized in the income statement of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of issuances of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sales of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlements of financial instrument classified as a derivative asset (liability) after deduction of derivative liability, measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfers of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset) into (out of) level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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