Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	$ 374,825,000	$ 425,681,000	$ 343,183,000	$ 372,903,000	$ 402,487,000	$ 406,950,000	$ 387,679,000	$ 356,545,000	$ 1,516,772,000	$ 1,553,661,000	$ 1,370,761,000
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)									1,132,399,000	1,151,117,000	979,573,000
Depreciation and depletion									94,575,000	87,127,000	100,023,000
Amortization											3,197,000
Accretion									13,189,000	12,469,000	12,499,000
Derivative financial instruments									(22,754,000)	(2,275,000)
Selling, general and administrative expenses									54,548,000	51,061,000	63,594,000
Other operating costs									2,949,000	1,419,000
Total costs and expenses									1,274,906,000	1,300,918,000	1,158,886,000
Operating income	54,482,000	81,634,000	58,722,000	47,027,000	63,984,000	60,173,000	74,706,000	53,882,000	241,866,000	252,743,000	211,875,000
Other income (expense)
Interest income									1,086,000	592,000	565,000
Interest expense									(36,327,000)	(33,866,000)	(46,938,000)
Other									(847,000)	(170,000)	157,000
Total other expense									(7,088,000)	(53,298,000)	(65,885,000)
Income from continuing operations before income tax provision and earnings (losses) from affiliates									234,778,000	199,445,000	145,990,000
Income tax (expense) benefit									(62,614,000)	(11,449,000)	(31,982,000)
Earnings from unconsolidated affiliates, net of tax									1,556,000	1,801,000	3,189,000
Other comprehensive income
Retiree medical plan amortization of prior service costs									1,575,000	1,305,000	1,405,000
Retiree medical plan adjustment									(4,665,000)	(5,602,000)	(3,587,000)
Decker pension adjustments									204,000	(1,885,000)	295,000
Income taxes on retiree medical plan adjustments									1,039,000	2,226,000	916,000
Other comprehensive income									(1,847,000)	(3,956,000)	(971,000)
Total comprehensive income attributable to controlling interest									171,873,000	185,841,000	32,112,000
Parent Company (CPE Resources)
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue										1,000	78,000
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)									(12,000)	192,000	221,000
Depreciation and depletion									2,403,000	2,027,000	2,080,000
Derivative financial instruments									(138,000)
Selling, general and administrative expenses									51,420,000	46,735,000	29,187,000
Total costs and expenses									53,673,000	48,954,000	31,488,000
Operating income									(53,673,000)	(48,953,000)	(31,410,000)
Other income (expense)
Interest income									1,086,000	590,000	558,000
Interest expense									(33,963,000)	(32,772,000)	(45,629,000)
Other											42,000
Total other expense									(32,877,000)	(32,182,000)	(45,029,000)
Income from continuing operations before income tax provision and earnings (losses) from affiliates									(86,550,000)	(81,135,000)	(76,439,000)
Income tax (expense) benefit									32,134,000	83,425,000	1,146,000
Earnings from unconsolidated affiliates, net of tax									17,000	21,000	26,000
Earnings (losses) from consolidated affiliates, net of tax									210,008,000	198,910,000	245,720,000
Net income									155,609,000	201,221,000	170,453,000
Other comprehensive income
Retiree medical plan amortization of prior service costs									1,575,000	1,305,000	1,405,000
Retiree medical plan adjustment									(4,665,000)	(5,602,000)	(3,587,000)
Decker pension adjustments									204,000	(1,885,000)	295,000
Income taxes on retiree medical plan adjustments									1,039,000	2,226,000
Other comprehensive income									(1,847,000)	(3,956,000)	(1,887,000)
Total comprehensive income attributable to controlling interest									153,762,000	197,265,000	168,566,000
Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									1,494,597,000	1,529,846,000	1,348,769,000
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)									1,103,013,000	1,125,340,000	959,432,000
Depreciation and depletion									89,554,000	80,689,000	97,189,000
Amortization											3,197,000
Accretion									9,852,000	8,539,000	9,591,000
Derivative financial instruments									(22,616,000)	(2,275,000)
Selling, general and administrative expenses									3,130,000	4,326,000	33,577,000
Other operating costs									2,949,000	1,419,000
Total costs and expenses									1,185,881,000	1,218,038,000	1,102,986,000
Operating income									308,715,000	311,808,000	245,783,000
Other income (expense)
Interest income										1,000
Interest expense									(1,991,000)	(932,000)	(1,230,000)
Other									(847,000)	(170,000)	115,000
Total other expense									(2,838,000)	(1,101,000)	(1,115,000)
Income from continuing operations before income tax provision and earnings (losses) from affiliates									305,877,000	310,707,000	244,668,000
Income tax (expense) benefit									(89,142,000)	(106,956,000)	(430,000)
Earnings from unconsolidated affiliates, net of tax									1,539,000	1,780,000	3,919,000
Earnings (losses) from consolidated affiliates, net of tax									(8,266,000)	(6,621,000)	(2,437,000)
Net income									210,008,000	198,910,000	245,720,000
Other comprehensive income
Retiree medical plan amortization of prior service costs									1,575,000	1,305,000	1,405,000
Retiree medical plan adjustment									(4,665,000)	(5,602,000)	(3,587,000)
Decker pension adjustments									204,000	(1,885,000)	295,000
Income taxes on retiree medical plan adjustments									1,039,000	2,225,000
Other comprehensive income									(1,847,000)	(3,957,000)	(1,887,000)
Total comprehensive income attributable to controlling interest									208,161,000	194,953,000	243,833,000
Non-Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									22,176,000	23,814,000	21,914,000
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)									29,397,000	25,585,000	19,920,000
Depreciation and depletion									2,618,000	4,411,000	754,000
Accretion									3,336,000	3,930,000	2,908,000
Selling, general and administrative expenses											782,000
Total costs and expenses									35,352,000	33,926,000	24,364,000
Operating income									(13,176,000)	(10,112,000)	(2,450,000)
Other income (expense)
Interest income										1,000	7,000
Interest expense									(63,000)	(58,000)	(58,000)
Total other expense									(62,000)	(57,000)	(51,000)
Income from continuing operations before income tax provision and earnings (losses) from affiliates									(13,238,000)	(10,169,000)	(2,501,000)
Income tax (expense) benefit									4,972,000	3,548,000	64,000
Net income									(8,266,000)	(6,621,000)	(2,437,000)
Other comprehensive income
Decker pension adjustments									204,000	(1,885,000)	295,000
Income taxes on retiree medical plan adjustments									(73,000)	678,000
Other comprehensive income									131,000	(1,207,000)	295,000
Total comprehensive income attributable to controlling interest									(8,135,000)	(7,828,000)	(2,142,000)
Eliminations
Other income (expense)
Earnings (losses) from consolidated affiliates, net of tax									(201,742,000)	(192,289,000)	(243,283,000)
Net income									(201,742,000)	(192,289,000)	(243,283,000)
Other comprehensive income
Retiree medical plan amortization of prior service costs									(1,575,000)	(1,305,000)	(1,405,000)
Retiree medical plan adjustment									4,665,000	5,602,000	3,587,000
Decker pension adjustments									(408,000)	3,770,000	(590,000)
Income taxes on retiree medical plan adjustments									(966,000)	(2,903,000)
Other comprehensive income									1,716,000	5,164,000	1,592,000
Total comprehensive income attributable to controlling interest									(200,026,000)	(187,125,000)	(241,691,000)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue									1,516,772,000	1,553,661,000	1,370,761,000
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)									1,132,399,000	1,151,117,000	979,573,000
Depreciation and depletion									94,575,000	87,127,000	100,023,000
Amortization											3,197,000
Accretion									13,189,000	12,469,000	12,499,000
Derivative financial instruments									(22,754,000)	(2,275,000)
Selling, general and administrative expenses									54,548,000	51,061,000	63,546,000
Other operating costs									2,949,000	1,419,000
Total costs and expenses									1,274,906,000	1,300,918,000	1,158,838,000
Operating income									241,866,000	252,743,000	211,923,000
Other income (expense)
Interest income									1,086,000	592,000	565,000
Interest expense									(36,016,000)	(33,762,000)	(46,917,000)
Other									(847,000)	(170,000)	157,000
Total other expense									(35,777,000)	(33,340,000)	(46,195,000)
Income from continuing operations before income tax provision and earnings (losses) from affiliates									206,089,000	219,403,000	165,728,000
Income tax (expense) benefit									(52,036,000)	(19,983,000)	780,000
Earnings from unconsolidated affiliates, net of tax									1,556,000	1,801,000	3,945,000
Net income									155,609,000	201,221,000	170,453,000
Other comprehensive income
Retiree medical plan amortization of prior service costs									1,575,000	1,305,000	1,405,000
Retiree medical plan adjustment									(4,665,000)	(5,602,000)	(3,587,000)
Decker pension adjustments									204,000	(1,885,000)	295,000
Income taxes on retiree medical plan adjustments									1,039,000	2,226,000
Other comprehensive income									(1,847,000)	(3,956,000)	(1,887,000)
Total comprehensive income attributable to controlling interest									$ 153,762,000	$ 197,265,000	$ 168,566,000