Stockholders' Equity (Details 2) (Western Coal Corp) In Millions, except Share data, unless otherwise specified	1 Months Ended
Apr. 30, 2011 Common Stock	Apr. 02, 2011 Common Stock USD ($)	Dec. 31, 2011 Warrants CAD
Stock issued in connection with the acquisition
Common shares issued (in shares)	8,951,558
Common shares issued value		$ 1,200.0
Warrants outstanding (in shares)			19,119
Exercise price (in dollars per share)			3.25

Value of equity interests (such as common shares, preferred shares, or partnership interest) issued or issuable to acquire the entity.

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-Name Statement of Financial Accounting Standard (FAS)

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-Paragraph 51

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

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-Name Emerging Issues Task Force (EITF)

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

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-Number 210

-Article 4

-Section 08

-Paragraph i

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

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

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-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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-Name Accounting Standards Codification

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