CONSOLIDATED STATEMENTS OF EQUITY (Cloud Peak Energy Inc. and Subsidiaries) (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings (Deficit)	Accumulated Other Comprehensive Income (Loss)	Non-controlling Interest	Comprehensive Income
Balances at Dec. 31, 2009	$ 445,478	$ 314		$ 251,083	$ 8,459	$ (6,951)	$ 192,573
Balances (in shares) at Dec. 31, 2009		31,449,000
Comprehensive income:
Net income	117,197				33,737		83,460	117,197
Postemployment benefit adjustment, net of tax	(971)					(1,625)	654	(971)
Total comprehensive income	116,226				33,737	(1,625)	84,114	116,226
Adjustment to beginning balance, tax agreement liability	2,414			2,414
Tax impact of Secondary Offering	(29,950)	294		242,222		(6,082)	(266,384)
Tax impact of Secondary Offering (in shares)		29,400,000
Stock compensation	7,234			7,234
Restricted stock issuance, net of forfeitures		1		(1)
Restricted stock issuance, net of forfeitures (in shares)		29,000
Distributions to Rio Tinto	(10,203)						(10,203)
Change in ownership allocation					100		(100)
Balances at Dec. 31, 2010	531,199	609		502,952	42,296	(14,658)
Balances (in shares) at Dec. 31, 2010		60,878,000
Comprehensive income:
Net income	189,797				189,797			189,797
Postemployment benefit adjustment, net of tax	(3,956)					(3,956)		(3,956)
Total comprehensive income	185,841				189,797	(3,956)		185,841
Tax impact of Secondary Offering	24,546			24,546
Stock compensation	8,803			8,803
Restricted stock issuance, net of forfeitures (in shares)		45,000
Balances at Dec. 31, 2011	750,389	609		536,301	232,093	(18,614)
Balances (in shares) at Dec. 31, 2011		60,923,000
Comprehensive income:
Net income	173,720				173,720			173,720
Postemployment benefit adjustment, net of tax	(1,847)					(1,847)		(1,847)
Total comprehensive income	171,873				173,720	(1,847)		171,873
Excess tax benefits related to equity-based compensation	1,201			1,201
Employee stock purchases	1,088	1		1,087
Employee stock purchases (in shares)		69,000
Stock compensation	11,796			11,796
Restricted stock issuance, net of forfeitures		1		(1)
Restricted stock issuance, net of forfeitures (in shares)		119,000
Employee common stock withheld to cover withholding taxes	(5,390)	(3)	(5,390)	3
Employee common stock withheld to cover withholding taxes (in shares)		(276,000)	276,000
Exercise of stock options	65			65
Exercise of stock options (in shares)		4,000
Balances at Dec. 31, 2012	$ 931,022	$ 608	$ (5,390)	$ 550,452	$ 405,813	$ (20,461)
Balances (in shares) at Dec. 31, 2012		60,839,000	276,000