Guarantees	9 Months Ended
Sep. 30, 2012
Guarantees [Abstract]
Guarantees	Guarantees On December 31, 2005, Arch entered into a purchase and sale agreement with Magnum Coal Company (“Magnum”) to sell certain assets to Magnum. On July 23, 2008, Patriot Coal Corporation acquired Magnum. 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 ("Bankruptcy Court"). The Company has agreed to continue to provide surety bonds and letters of credit for certain Magnum obligations, primarily reclamation. The surety bonding amounts are mandated by the state and are not directly related to the estimated cost to reclaim the properties. At September 30, 2012, the Company had $35.3 million of surety bonds remaining related to Magnum properties , however Patriot Coal has posted letters of credit of $16.7 million in the Company’s favor. On September 20, 2012, Patriot filed a motion in the Bankruptcy Court to reject a master coal sales and services agreement entered into on December 31, 2005 between Arch and Magnum that was established to supply coal for a sales agreement with a customer who did not consent to the assignment of their contract to Magnum. The motion is pending before the court. The underlying coal sales agreement provides the coal supplier with the ability to either ship coal or to elect to buy out of its monthly obligations at amounts that are predetermined for the remainder of the agreement. The remaining monthly buyout election payments total approximately $64 million, payable if elected through the end of the contract in 2017. If the Bankruptcy Court approves the rejection of the master coal sales agreement, the Company believes it would have the right to source the underlying contract from a variety of alternative sources, including the potential of sourcing it from its operations. The Company could elect to buy out of the contract obligations, depending on market conditions. The Company does not expect that, if elected, the monthly buyout payments would have a material impact on the Company's liquidity. Additionally, a predecessor of the Company entered into a guarantee for the delivery of coal under a contract assigned to Magnum. Patriot's motion to reject the contract has been approved by the Bankruptcy Court. If the guarantee is enforceable against the Company, then it may be required to fulfill Magnum’s delivery or payment obligations. The Company does not expect that fulfilling the guarantee would have a material impact on the Company's liquidity. Because the Company does not believe that it is probable that it would have to purchase replacement coal to fulfill the customer contract or perform under the guarantee, no losses have been recorded in the consolidated financial statements as of September 30, 2012. Should Patriot not emerge from bankruptcy, or is incapable of paying retiree medical benefits pursuant to Section 9711 of the Coal Industry Retiree Health Benefit Act of 1992 (“Coal Act”) to a certain subset of retirees, the Company could become responsible for their retiree medical obligations. The retirees were employees of certain subsidiaries sold to Magnum and their predecessor entities who retired prior to October 1, 1994. The Company does not have the information necessary to determine the potential amount of such obligations, but does not expect that the annual benefit payments would have a material impact on the Company's liquidity.

Guarantees

On December 31, 2005, Arch entered into a purchase and sale agreement with Magnum Coal Company (“Magnum”) to sell certain assets to Magnum. On July 23, 2008, Patriot Coal Corporation acquired Magnum. 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 ("Bankruptcy Court").

The Company has agreed to continue to provide surety bonds and letters of credit for certain Magnum obligations, primarily reclamation. The surety bonding amounts are mandated by the state and are not directly related to the estimated cost to reclaim the properties. At September 30, 2012, the Company had $35.3 million of surety bonds remaining related to Magnum properties , however Patriot Coal has posted letters of credit of $16.7 million in the Company’s favor.

On September 20, 2012, Patriot filed a motion in the Bankruptcy Court to reject a master coal sales and services agreement entered into on December 31, 2005 between Arch and Magnum that was established to supply coal for a sales agreement with a customer who did not consent to the assignment of their contract to Magnum. The motion is pending before the court. The underlying coal sales agreement provides the coal supplier with the ability to either ship coal or to elect to buy out of its monthly obligations at amounts that are predetermined for the remainder of the agreement. The remaining monthly buyout election payments total approximately $64 million, payable if elected through the end of the contract in 2017. If the Bankruptcy Court approves the rejection of the master coal sales agreement, the Company believes it would have the right to source the underlying contract from a variety of alternative sources, including the potential of sourcing it from its operations. The Company could elect to buy out of the contract obligations, depending on market conditions. The Company does not expect that, if elected, the monthly buyout payments would have a material impact on the Company's liquidity.

Additionally, a predecessor of the Company entered into a guarantee for the delivery of coal under a contract assigned to Magnum. Patriot's motion to reject the contract has been approved by the Bankruptcy Court. If the guarantee is enforceable against the Company, then it may be required to fulfill Magnum’s delivery or payment obligations. The Company does not expect that fulfilling the guarantee would have a material impact on the Company's liquidity.

Because the Company does not believe that it is probable that it would have to purchase replacement coal to fulfill the customer contract or perform under the guarantee, no losses have been recorded in the consolidated financial statements as of September 30, 2012.

Should Patriot not emerge from bankruptcy, or is incapable of paying retiree medical benefits pursuant to Section 9711 of the Coal Industry Retiree Health Benefit Act of 1992 (“Coal Act”) to a certain subset of retirees, the Company could become responsible for their retiree medical obligations. The retirees were employees of certain subsidiaries sold to Magnum and their predecessor entities who retired prior to October 1, 1994. The Company does not have the information necessary to determine the potential amount of such obligations, but does not expect that the annual benefit payments would have a material impact on the Company's liquidity.