Earning Per Share (CPE Inc. only) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended						1 Months Ended		12 Months Ended				1 Months Ended				3 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES numerator denominator	Dec. 31, 2010 CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES	Dec. 31, 2009 CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES	Dec. 31, 2010 CPE Inc	Nov. 30, 2009 CPE Inc	Dec. 31, 2011 CPE Inc	Dec. 31, 2010 CPE Inc	Dec. 31, 2009 CPE Inc	Dec. 31, 2010 Rio Tinto Energy America (RTEA)	Dec. 31, 2010 Rio Tinto Energy America (RTEA) CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES	Dec. 31, 2010 CPE Resources LLC CPE Inc	Nov. 30, 2009 CPE Resources LLC CPE Inc	Nov. 19, 2009 CPE Resources LLC Rio Tinto America and other Rio Tinto affiliates	Dec. 31, 2010 Scenario, previously reported Clerical error	Dec. 31, 2010 Scenario, previously reported Clerical error
Number of shares held by RTEA																				1
Total common membership units issued and outstanding																								60,000,000
Ratio between the number of common membership units in LLC and the number of common stock of parent company issued and outstanding and not held in treasury, numerator												1
Ratio between the number of common membership units in LLC and the number of common stock of parent company issued and outstanding and not held in treasury, denominator												1
Percentage of common membership units acquired to units outstanding																							51.00%
Common stock issued to public (in shares)																30,600,000
Common stock were outstanding during periods prior to IPO for purposes of calculating basic and diluted earnings per share (in shares)																	60,000,000
Secondary Offering shares of common stock on behalf of Rio Tinto															29,400,000
Percentage of divestiture of Rio Tinto's holdings in LLC																					100.00%
Common membership units acquired from RTEA (in shares)																						29,400,000	30,600,000
Amounts attributable to controlling interest common stockholders:
Income from continuing operations									$ 189,797	$ 33,737	$ 170,623	$ 201,221	$ 170,453	$ 186,688			$ 189,797	$ 33,737	$ 170,623
Income from discontinued operations											211,078			211,078					211,078
Net income					$ 12,881	$ (6,599)	$ 15,858	$ 11,597	$ 189,797	$ 33,737	$ 381,701
Income from continuing operations (in dollars per share)	$ 0.73	$ 0.41	$ 1.58	$ 0.45	$ 0.36	$ (0.22)	$ 0.52	$ 0.38	$ 3.16	$ 1.06	$ 3.01						$ 3.16	$ 1.06	$ 3.01						$ 0.28	$ 0.98
Earnings per share from discontinued operations (in dollars per share)											$ 3.73								$ 3.73
Basic earnings per share (in dollars per share)									$ 3.16	$ 1.06	$ 6.74						$ 3.16	$ 1.06	$ 6.74
Weighted-average shares outstanding (in shares)					35,700,000				60,004,000	31,889,000	56,617,000						60,004,000	31,889,000	56,617,000						45,300,000	34,300,000
Combined federal and state statutory rate (as a percent)									36.00%			35.00%	35.00%	35.00%			35.00%	35.00%	35.00%