Settlement with Patriot Coal	12 Months Ended
Dec. 31, 2013
Settlement with Patriot Coal [Abstract]
Legal Matters and Contingencies [Text Block]	Settlement with Patriot Coal On December 31, 2005, Arch entered into a purchase and sale agreement to sell mining complexes to Magnum Coal Company ("Magnum"). On July 23, 2008, Patriot Coal Corporation acquired Magnum from Arc Light Capital Partners. On July 9, 2012, Patriot Coal Corporation and certain of its wholly owned subsidiaries, including Magnum, (collectively, "Patriot") filed voluntary petitions for reorganization under Chapter 11 of the U.S. Code in the U.S. Bankruptcy Court for the Southern District of New York. On October 4, 2013, we entered into a term sheet that was approved by the U.S. Bankruptcy Court on November 7, 2013, that resolves all pending and potential legal claims with Patriot stemming from the Company's sale of mining complexes to Magnum and the subsequent purchase of those companies by Patriot in 2008. The Company paid $5.0 million in cash to Patriot upon its exit from bankruptcy, which is reflected in "Other operating expense (income), net" in the consolidated statement of operations for the year ended December 31, 2013. Additionally, the settlement includes the release of a $16.7 million letter of credit posted by Patriot in the Company's favor for surety bonds related to the companies sold to Magnum. The Company also purchased Patriot's Guffey reserves for $16.0 million in cash upon their exit from bankruptcy. The Guffey reserves border the Company's Leer metallurgical coal complex.

Settlement with Patriot Coal

On December 31, 2005, Arch entered into a purchase and sale agreement to sell mining complexes to Magnum Coal Company ("Magnum"). On July 23, 2008, Patriot Coal Corporation acquired Magnum from Arc Light Capital Partners. On July 9, 2012, Patriot Coal Corporation and certain of its wholly owned subsidiaries, including Magnum, (collectively, "Patriot") filed voluntary petitions for reorganization under Chapter 11 of the U.S. Code in the U.S. Bankruptcy Court for the Southern District of New York.

On October 4, 2013, we entered into a term sheet that was approved by the U.S. Bankruptcy Court on November 7, 2013, that resolves all pending and potential legal claims with Patriot stemming from the Company's sale of mining complexes to Magnum and the subsequent purchase of those companies by Patriot in 2008.

The Company paid $5.0 million in cash to Patriot upon its exit from bankruptcy, which is reflected in "Other operating expense (income), net" in the consolidated statement of operations for the year ended December 31, 2013. Additionally, the settlement includes the release of a $16.7 million letter of credit posted by Patriot in the Company's favor for surety bonds related to the companies sold to Magnum. The Company also purchased Patriot's Guffey reserves for $16.0 million in cash upon their exit from bankruptcy. The Guffey reserves border the Company's Leer metallurgical coal complex.