Fair Value Measurements, Level 3 Fair Value Input Reconciliation Table (Details) (Derivative [Member], Midwest premium swap contracts, USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013
Derivative [Member] \| 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	0.2
Sales	0
Issuances	0
Settlements	(0.3)
Transactions involving Level 3 derivatives — net	(0.1)
Transfers in and (or) out of Level 3 valuation hierarchy	0
Balance at June 30, 2013	0.5
Total gains included in Unrealized losses (gains) on derivative instruments, attributable to the change in unrealized gains/losses relating to derivative contracts held at June 30, 2013:	$ 0.2

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

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- Definition

Amount of gain (loss) recognized in earnings, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

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- Definition

Issues that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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- Definition

Purchases that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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- Definition

Amount of purchases, sales, issues, and settlements (net) which have taken place in relation to assets measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Sales that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Settlements that have taken place during the period in relation to assets measured at fair value and categorized within Level 3 of the fair value hierarchy.

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- Definition

This element represents [net] transfers in to and out of assets measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(3)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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