Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Current assets
Cash and cash equivalents	$ 197,691	$ 404,240	$ 340,101	$ 268,316
Investments in marketable securities	80,341	75,228
Restricted cash		71,245
Accounts receivable	76,117	95,247
Due from related parties	1,561	471
Inventories, net	81,675	71,648
Deferred income taxes	28,112	37,528
Derivative assets	16,107	2,275
Other assets	16,513	13,019
Total current assets	495,795	770,901
Noncurrent assets
Property, plant and equipment, net	1,678,294	1,350,135
Goodwill	35,634	35,634
Deferred income taxes	101,075	132,828
Investments and other assets	35,510	29,773
Total assets	2,351,323	2,319,319
Current liabilities
Accounts payable and accrued expenses	51,494	71,377
Royalties and production taxes	129,351	136,072
Accrued expenses	50,364	65,928
Due to related parties		27,420
Current portion of federal coal lease obligations	63,191	102,198
Other liabilities	2,770	4,971
Total current liabilities	314,750	399,709
Noncurrent liabilities
Senior notes	596,506	596,077
Federal coal lease obligations, net of current portion	122,928	186,119
Asset retirement obligations, net of current portion	238,991	192,707
Other liabilities	50,073	42,795
Total liabilities	1,420,301	1,568,930
Commitments and Contingencies (Note 15)
Total members' equity	931,022	750,389
Total liabilities and members' equity	2,351,323	2,319,319
Parent Company (CPE Resources)
Current assets
Cash and cash equivalents	195,076	401,087	322,010	246,470
Investments in marketable securities	80,341	75,228
Restricted cash		71,245
Accounts receivable		130
Inventories, net	6,741	5,753
Derivative assets	138
Other assets	7
Total current assets	282,303	553,443
Noncurrent assets
Property, plant and equipment, net	9,239	6,684
Deferred income taxes	22,807	34,307
Investments and other assets	1,682,267	1,134,791
Total assets	1,996,616	1,729,225
Current liabilities
Accounts payable and accrued expenses	2,558	2,801
Accrued expenses	2,087	2,094
Due to related parties	396,137	282,661
Other liabilities	76	45
Total current liabilities	400,858	287,601
Noncurrent liabilities
Senior notes	596,506	596,077
Other liabilities	61	119
Total liabilities	997,425	883,797
Commitments and Contingencies (Note 15)
Total members' equity	999,190	845,428
Total liabilities and members' equity	1,996,615	1,729,225
Guarantor Subsidiaries
Current assets
Cash and cash equivalents		2	4
Accounts receivable	74,008	92,936
Due from related parties	385,102	256,460
Inventories, net	71,312	61,677
Deferred income taxes	21,124	30,648
Derivative assets	13,647	2,275
Other assets	16,100	12,610
Total current assets	581,293	456,608
Noncurrent assets
Property, plant and equipment, net	1,666,020	1,338,839
Goodwill	35,634	35,634
Deferred income taxes	24,650	28,931
Total assets	2,307,597	1,860,012
Current liabilities
Accounts payable and accrued expenses	45,896	66,850
Royalties and production taxes	126,726	133,349
Accrued expenses	41,529	44,677
Current portion of federal coal lease obligations	63,191	102,198
Other liabilities	1,754	3,960
Total current liabilities	279,096	351,034
Noncurrent liabilities
Federal coal lease obligations, net of current portion	122,928	186,119
Asset retirement obligations, net of current portion	164,626	126,267
Other liabilities	77,655	84,201
Total liabilities	644,305	747,622
Commitments and Contingencies (Note 15)
Total members' equity	1,663,293	1,112,390
Total liabilities and members' equity	2,307,598	1,860,012
Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	2,615	3,151	18,086	21,846
Accounts receivable	2,108	2,181
Due from related parties	42
Inventories, net	3,622	4,218
Other assets	117
Total current assets	8,504	9,550
Noncurrent assets
Property, plant and equipment, net	3,035	4,612
Deferred income taxes	18,679	15,042
Investments and other assets	4,470
Total assets	34,688	29,204
Current liabilities
Accounts payable and accrued expenses	1,117	1,726
Royalties and production taxes	2,625	2,723
Accrued expenses	292	5,028
Due to related parties		1,219
Other liabilities	966	966
Total current liabilities	5,001	11,662
Noncurrent liabilities
Asset retirement obligations, net of current portion	74,365	66,440
Other liabilities	5,806	6,021
Total liabilities	85,172	84,123
Commitments and Contingencies (Note 15)
Total members' equity	(50,484)	(54,919)
Total liabilities and members' equity	34,688	29,204
Eliminations
Current assets
Due from related parties	(385,144)	(256,460)
Deferred income taxes	(28)
Total current assets	(385,171)	(256,460)
Noncurrent assets
Investments and other assets	(1,646,259)	(1,105,018)
Total assets	(2,031,430)	(1,361,478)
Current liabilities
Due to related parties	(385,144)	(256,460)
Other liabilities	(27)
Total current liabilities	(385,172)	(256,460)
Noncurrent liabilities
Other liabilities	(33,449)	(47,546)
Total liabilities	(418,621)	(304,006)
Commitments and Contingencies (Note 15)
Total members' equity	(1,612,809)	(1,057,472)
Total liabilities and members' equity	(2,031,430)	(1,361,478)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Current assets
Cash and cash equivalents	197,691	404,240	340,100	268,316
Investments in marketable securities	80,341	75,228
Restricted cash		71,245
Accounts receivable	76,117	95,247
Inventories, net	81,675	71,648
Deferred income taxes	21,096	30,648
Derivative assets	13,785	2,275
Other assets	16,224	12,610
Total current assets	486,929	763,141
Noncurrent assets
Property, plant and equipment, net	1,678,294	1,350,135
Goodwill	35,634	35,634
Deferred income taxes	66,136	78,280
Investments and other assets	40,478	29,773
Total assets	2,307,471	2,256,963
Current liabilities
Accounts payable and accrued expenses	49,571	71,377
Royalties and production taxes	129,351	136,072
Accrued expenses	43,908	51,799
Due to related parties	10,993	27,420
Current portion of federal coal lease obligations	63,191	102,198
Other liabilities	2,769	4,971
Total current liabilities	299,783	393,837
Noncurrent liabilities
Senior notes	596,506	596,077
Federal coal lease obligations, net of current portion	122,928	186,119
Asset retirement obligations, net of current portion	238,991	192,707
Other liabilities	50,073	42,795
Total liabilities	1,308,281	1,411,535
Commitments and Contingencies (Note 15)
Total members' equity	999,190	845,428
Total liabilities and members' equity	$ 2,307,471	$ 2,256,963