CPE Inc. (Parent Company Only) Financial Information (Details) (USD $) In Thousands, unless otherwise specified	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
Condensed Statements of Operations
Management fee	$ 374,825	$ 425,681	$ 343,183	$ 372,903	$ 402,487	$ 406,950	$ 387,679	$ 356,545	$ 1,516,772	$ 1,553,661	$ 1,370,761
Costs and expenses
Total costs and expenses									1,274,906	1,300,918	1,158,886
Income (Loss) before equity in earnings of consolidated subsidiaries and income taxes									234,778	199,445	145,990
Interest expense									(36,327)	(33,866)	(46,938)
Change in tax agreement liability							(42,700)		29,000	(19,854)	(19,669)
Income tax (expense) benefit									(62,614)	(11,449)	(31,982)
Net income attributable to controlling interest									173,720	189,797	33,737
Current assets
Deferred income taxes	28,112				37,528				28,112	37,528
Due from related party	1,561				471				1,561	471
Other assets	16,513				13,019				16,513	13,019
Total current assets	495,795				770,901				495,795	770,901
Deferred income taxes	101,075				132,828				101,075	132,828
Other assets	40,525				29,821				40,525	29,821
Total assets	2,351,323				2,319,319				2,351,323	2,319,319
Current liabilities
Accounts payable	49,589				71,427				49,589	71,427
Current portion of tax agreement liability	19,485				19,113				19,485	19,113
Accrued expenses	50,364				65,928				50,364	65,928
Total current liabilities	314,750				399,709				314,750	399,709
Tax agreement liability, net of current portion	97,053				151,523				97,053	151,523
Total liabilities	1,420,301				1,568,930				1,420,301	1,568,930
Total liabilities and equity	2,351,323				2,319,319				2,351,323	2,319,319
Cloud Peak Energy Inc.
Condensed Statements of Operations
Management fee									8,076	7,254	9,469
Costs and expenses
General and administrative expenses									8,076	7,254	9,517
Total costs and expenses									8,076	7,254	9,517
Income (Loss) before equity in earnings of consolidated subsidiaries and income taxes											(48)
Equity in earnings of CPE Resources									155,609	201,221	86,237
Interest expense									(312)	(104)	(21)
Change in tax agreement liability									29,000	(19,854)	(19,669)
Income tax (expense) benefit									(62,614)	(11,449)	(31,982)
Net income attributable to controlling interest									173,720	189,797	33,737
Current assets
Deferred income taxes	7,015				6,881				7,015	6,881
Due from related party	12,554				27,892				12,554	27,892
Other assets	289				409				289	409
Total current assets	19,858				35,182				19,858	35,182
Deferred income taxes	34,939				54,548				34,939	54,548
Other assets	48				48				48	48
Investment in CPE Resources	999,190				845,428				999,190	845,428
Total assets	1,054,035				935,206				1,054,035	935,206
Current liabilities
Accounts payable	18				50				18	50
Current portion of tax agreement liability	19,485				19,113				19,485	19,113
Accrued expenses	6,457				14,131				6,457	14,131
Total current liabilities	25,960				33,294				25,960	33,294
Tax agreement liability, net of current portion	97,053				151,523				97,053	151,523
Total liabilities	123,013				184,817				123,013	184,817
Total stockholders equity	931,022				750,389				931,022	750,389
Total liabilities and equity	$ 1,054,035				$ 935,206				$ 1,054,035	$ 935,206