Basis Of Presentation (Policy)	6 Months Ended
Jun. 30, 2012
Basis Of Presentation [Abstract]
Basis Of Presentation	1. Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the accounts of Arch Coal, Inc. and its subsidiaries and controlled entities (the “Company”). The Company’s primary business is the production of steam and metallurgical coal from surface and underground mines located throughout the United States, for sale to utility, industrial and export markets. On June 15, 2011, the Company acquired International Coal Group, Inc. (“ICG”). The Company currently operates 18 mining complexes in West Virginia, Kentucky, Maryland, Virginia, Illinois, Wyoming, Colorado and Utah. All subsidiaries (except as noted below) are wholly-owned. Intercompany transactions and accounts have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial reporting and U.S. Securities and Exchange Commission regulations. In the opinion of management, all adjustments, consisting of normal, recurring accruals considered necessary for a fair presentation, have been included. Results of operations for the three and six month periods ended June 30, 2012 are not necessarily indicative of results to be expected for the year ending December 31, 2012. These financial statements should be read in conjunction with the audited financial statements and related notes as of and for the year ended December 31, 2011 included in the Company’s Annual Report on Form 10-K/A filed with the U.S. Securities and Exchange Commission. The Company owned a 99% membership interest and acted as the managing member in Arch Western Resources, LLC (“Arch Western”) a joint venture with Delta Housing, Inc., a subsidiary of BP p.l.c, Arch Western operates coal mines in Wyoming, Colorado and Utah. On April 9, 2012, Delta Housing, Inc. exercised their contractual right to require us to purchase their membership interests in Arch Western. The negotiated purchase amount of $17.5 million was paid on July 2, 2012.

The accompanying unaudited condensed consolidated financial statements include the accounts of Arch Coal, Inc. and its subsidiaries and controlled entities (the “Company”). The Company’s primary business is the production of steam and metallurgical coal from surface and underground mines located throughout the United States, for sale to utility, industrial and export markets. On June 15, 2011, the Company acquired International Coal Group, Inc. (“ICG”). The Company currently operates 18 mining complexes in West Virginia, Kentucky, Maryland, Virginia, Illinois, Wyoming, Colorado and Utah. All subsidiaries (except as noted below) are wholly-owned. Intercompany transactions and accounts have been eliminated in consolidation.

The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial reporting and U.S. Securities and Exchange Commission regulations. In the opinion of management, all adjustments, consisting of normal, recurring accruals considered necessary for a fair presentation, have been included. Results of operations for the three and six month periods ended June 30, 2012 are not necessarily indicative of results to be expected for the year ending December 31, 2012. These financial statements should be read in conjunction with the audited financial statements and related notes as of and for the year ended December 31, 2011 included in the Company’s Annual Report on Form 10-K/A filed with the U.S. Securities and Exchange Commission.

The Company owned a 99% membership interest and acted as the managing member in Arch Western Resources, LLC (“Arch Western”) a joint venture with Delta Housing, Inc., a subsidiary of BP p.l.c, Arch Western operates coal mines in Wyoming, Colorado and Utah. On April 9, 2012, Delta Housing, Inc. exercised their contractual right to require us to purchase their membership interests in Arch Western. The negotiated purchase amount of $17.5 million was paid on July 2, 2012.