CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Cloud Peak Energy Resources LLC and Subsidiaries) (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
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, net									(847,000)	(170,000)	157,000
Total other expense									(7,088,000)	(53,298,000)	(65,885,000)
Income before income tax provision and earnings from unconsolidated 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 tax on retiree medical plan and pension 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
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
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, net									(847,000)	(170,000)	157,000
Total other expense									(35,777,000)	(33,340,000)	(46,195,000)
Income before income tax provision and earnings from unconsolidated 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 tax on retiree medical plan and pension 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