Organization and Business (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended		0 Months Ended				0 Months Ended		0 Months Ended			12 Months Ended
	Dec. 31, 2012 T item	Dec. 05, 2012	Dec. 15, 2010 CPE Resources	Dec. 31, 2012 Ambre	Dec. 05, 2012 Ambre	Dec. 05, 2012 Ambre Scenario forecast	Dec. 05, 2012 Ambre Scenario forecast Maximum	Dec. 31, 2012 Arch Coal	Nov. 19, 2009 Cloud Peak Energy Inc.	Nov. 19, 2009 Cloud Peak Energy Inc. CPE Resources LLC	Dec. 15, 2010 Rio Tinto CPE Resources LLC	Dec. 31, 2012 Wyoming item
Equity details
Number of surface coal mines operated by entity	3
Number of major development projects	2
Ownership interest in Decker (as a percent)	50.00%	50.00%			50.00%
Number of largest coal mines operated by entity												2
Number of largest coal mines in region												4
Percentage of electricity generated from the coal produced	4.00%
Capacity of proven and probable reserves controlled	1,300,000,000
Amount of outstanding reclamation and lease bonds for the Decker Mine considered for replacement						$ 70.7
Cash component of consideration for the Decker interests						57
Issuance of promissory note						64
Throughput capacity per year for which option is expected to be granted							5,000,000
Ownership percentage in Millennium Bulk Terminals coal export facility				62.00%				38.00%
Initial term of throughput capacity	10 years
Throughput capacity, number of renewal options	4
Renewal term of throughput capacity	5 years
Expected deconsolidation impact of the Decker accounts on ARO liability	$ 74.4
Secondary Offering
Percentage of common membership units acquired to units outstanding									51.00%	51.00%
Remaining interest in CPE Resources											49.00%
Percentage of ownership interest acquired in subsidiary			100.00%