Property Transactions	12 Months Ended
Property Transactions	Dec. 31, 2011
Property Transactions [Abstract]
Property Transactions	2. Property Transactions On December 14, 2011, the Company was awarded a federal coal lease for the South Hilight tract in Wyoming for a price of $300.0 million. The bid price will be paid in five equal installments, with the first one made in December 2011. The coal lease will give the Company the right to mine an estimated 222 million tons of coal reserves. The South Hilight tract is contiguous to the Company's Black Thunder mining complex. On November 12, 2009, the Company entered into a lease of coal reserves and other coal resources from Great Northern Properties Limited Partnership in Montana for $73.1 million. On March 18, 2010, the Company was awarded a Montana state coal lease for the Otter Creek tracts for a price of $85.8 million. These two transactions gave the Company the right to mine approximately 1.4 billion tons of coal reserves in the Montana's Otter Creek area. The total of the Company's future lease bonus payments due are $23.4 million in 2012, $83.4 million in 2013, $67.3 million in 2014, $60.0 million in 2015 and $60.0 million in 2016.

The entire disclosure for long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. This disclosure may include property plant and equipment accounting policies and methodology, a schedule of property, plant and equipment gross, additions, deletions, transfers and other changes, depreciation, depletion and amortization expense, net, accumulated depreciation, depletion and amortization expense and useful lives, income statement disclosures, assets held for sale and public utility disclosures.

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