CONSOLIDATED STATEMENTS OF EQUITY (Cloud Peak Energy Resources LLC and Subsidiaries) (USD $) In Thousands, unless otherwise specified	Total	Accumulated Other Comprehensive Income (Loss)	Comprehensive Income	CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES	CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES Accumulated Other Comprehensive Income (Loss)	CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES Comprehensive Income	CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES Managing Member's Equity	CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES Rio Tinto Member's Equity
Balances at Dec. 31, 2009				$ 398,569	$ (21,016)		$ 216,857	$ 202,728
Comprehensive income:
Net income				170,453		170,453	86,993	83,460
Postemployment benefit adjustment, net of tax	(971)	(1,625)	(971)	(1,887)	(1,887)	(1,887)
Total comprehensive income attributable to controlling interest	32,112			168,566	(1,887)	168,566	86,993	83,460
Adjustment to beginning balance, deferred tax asset				1,895			980	915
Tax impact of Secondary Offering	(29,950)	(6,082)					276,801	(276,801)
Distributions to Rio Tinto	(10,203)			(21,129)			(10,926)	(10,203)
Change in ownership allocation							99	(99)
Establishment of deferred taxes				91,358	8,245		83,113
Balances at Dec. 31, 2010				639,259	(14,658)		653,917
Comprehensive income:
Net income				201,221		201,221	201,221
Postemployment benefit adjustment, net of tax	(3,956)	(3,956)	(3,956)	(3,956)	(3,956)	(3,956)
Total comprehensive income attributable to controlling interest	185,841			197,265	(3,956)	197,265	201,221
Tax impact of Secondary Offering	24,546			9,066			9,066
Distributions to Rio Tinto				(162)			(162)
Balances at Dec. 31, 2011				845,428	(18,614)		864,042
Comprehensive income:
Net income				155,609		155,609	155,609
Postemployment benefit adjustment, net of tax	(1,847)	(1,847)	(1,847)	(1,847)	(1,847)	(1,847)
Total comprehensive income attributable to controlling interest	171,873			153,762	(1,847)	153,762	155,609
Balances at Dec. 31, 2012				$ 999,190	$ (20,461)		$ 1,019,651