Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Current assets
Cash and cash equivalents	$ 199,747	$ 197,691	$ 121,584	$ 404,240
Investments in marketable securities	80,501	80,341
Accounts receivable	95,886	76,117
Due from related parties	836	1,561
Inventories, net	82,659	81,675
Deferred income taxes	26,538	28,112
Derivative financial instruments	37,252	13,785
Other assets	26,737	16,513
Total current assets	550,156	495,795
Noncurrent assets
Property, plant and equipment, net	1,679,731	1,678,294
Goodwill	35,634	35,634
Deferred income taxes	91,909	101,075
Other assets	44,259	40,525
Total assets	2,401,689	2,351,323
Current liabilities
Accounts payable	62,697	49,589
Royalties and production taxes	129,467	129,351
Accrued expenses	58,570	50,364
Due to related parties	6,299	12,554
Current portion of federal coal lease obligations	54,339	63,191
Other liabilities	3,854	2,770
Total current liabilities	328,412	314,750
Noncurrent liabilities
Senior notes	596,735	596,506
Federal coal lease obligations, net of current portion	122,928	122,928
Asset retirement obligations, net of current portion	239,576	238,991
Other liabilities	61,846	50,073
Total liabilities	1,446,550	1,420,301
Commitments and Contingencies (Note 9)
Total member's equity	955,139	931,022
Total liabilities and member's equity	2,401,689	2,351,323
Parent Company (CPE Resources)
Current assets
Cash and cash equivalents	198,394	195,076	119,131	401,087
Investments in marketable securities	80,501	80,341
Inventories, net	5,880	6,741
Derivative financial instruments		138
Other assets		7
Total current assets	284,775	282,303
Noncurrent assets
Property, plant and equipment, net	9,458	9,239
Deferred income taxes	27,143	22,807
Other assets	1,732,560	1,682,267
Total assets	2,053,936	1,996,616
Current liabilities
Accounts payable	3,004	2,558
Accrued expenses	2,101	2,087
Due to related parties	432,105	396,137
Current deferred income taxes	27	27
Other liabilities	51	49
Total current liabilities	437,288	400,858
Noncurrent liabilities
Senior notes	596,735	596,506
Other liabilities	31	61
Total liabilities	1,034,054	997,425
Total member's equity	1,019,882	999,190
Total liabilities and member's equity	2,053,936	1,996,615
Guarantor Subsidiaries
Current assets
Cash and cash equivalents	208		2	2
Accounts receivable	17,392	74,008
Due from related parties	492,762	385,102
Inventories, net	72,658	71,312
Deferred income taxes	19,550	21,124
Derivative financial instruments	37,252	13,647
Other assets	21,183	16,100
Total current assets	661,005	581,293
Noncurrent assets
Property, plant and equipment, net	1,667,437	1,666,020
Goodwill	35,634	35,634
Deferred income taxes	11,711	24,650
Total assets	2,375,787	2,307,597
Current liabilities
Accounts payable	56,383	45,896
Royalties and production taxes	127,725	126,726
Accrued expenses	49,178	41,529
Current portion of federal coal lease obligations	54,339	63,191
Other liabilities	2,837	1,754
Total current liabilities	290,462	279,096
Noncurrent liabilities
Federal coal lease obligations, net of current portion	122,928	122,928
Asset retirement obligations, net of current portion	167,884	164,626
Other liabilities	79,134	77,655
Total liabilities	660,408	644,305
Total member's equity	1,715,379	1,663,293
Total liabilities and member's equity	2,375,787	2,307,598
Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	1,145	2,615	2,451	3,151
Accounts receivable	78,494	2,108
Due from related parties		42
Inventories, net	4,121	3,622
Other assets	150	117
Total current assets	83,910	8,504
Noncurrent assets
Property, plant and equipment, net	2,836	3,035
Deferred income taxes	18,116	18,679
Other assets		4,470
Total assets	104,862	34,688
Current liabilities
Accounts payable	3,310	1,117
Royalties and production taxes	1,742	2,625
Accrued expenses	5,380	292
Due to related parties	66,120
Other liabilities	966	966
Total current liabilities	77,518	5,001
Noncurrent liabilities
Asset retirement obligations, net of current portion	71,691	74,365
Other liabilities	5,899	5,806
Total liabilities	155,108	85,172
Total member's equity	(50,246)	(50,484)
Total liabilities and member's equity	104,862	34,688
Eliminations
Current assets
Due from related parties	(491,926)	(383,582)
Deferred income taxes	(27)	(28)
Total current assets	(491,953)	(383,610)
Noncurrent assets
Other assets	(1,688,350)	(1,646,259)
Total assets	(2,180,303)	(2,029,869)
Current liabilities
Due to related parties	(491,926)	(383,583)
Current deferred income taxes	(27)	(27)
Total current liabilities	(491,953)	(383,611)
Noncurrent liabilities
Other liabilities	(23,217)	(33,449)
Total liabilities	(515,170)	(417,060)
Total member's equity	(1,665,133)	(1,612,809)
Total liabilities and member's equity	(2,180,303)	(2,029,869)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Current assets
Cash and cash equivalents	199,747	197,691	121,584	404,240
Investments in marketable securities	80,501	80,341
Accounts receivable	95,886	76,117
Due from related parties	836	1,561
Inventories, net	82,659	81,675
Deferred income taxes	19,523	21,096
Derivative financial instruments	37,252	13,785
Other assets	21,333	16,224
Total current assets	537,737	488,490
Noncurrent assets
Property, plant and equipment, net	1,679,731	1,678,294
Goodwill	35,634	35,634
Deferred income taxes	56,970	66,136
Other assets	44,210	40,478
Total assets	2,354,282	2,309,032
Current liabilities
Accounts payable	62,697	49,571
Royalties and production taxes	129,467	129,351
Accrued expenses	56,659	43,908
Due to related parties	6,299	12,554
Current portion of federal coal lease obligations	54,339	63,191
Other liabilities	3,854	2,769
Total current liabilities	313,315	301,344
Noncurrent liabilities
Senior notes	596,735	596,506
Federal coal lease obligations, net of current portion	122,928	122,928
Asset retirement obligations, net of current portion	239,576	238,991
Other liabilities	61,846	50,073
Total liabilities	1,334,400	1,309,842
Commitments and Contingencies (Note 9)
Total member's equity	1,019,882	999,190
Total liabilities and member's equity	$ 2,354,282	$ 2,309,032