Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Current assets
Cash and cash equivalents	$ 218,604	$ 197,691	$ 488,040	$ 404,240
Investments in marketable securities	80,473	80,341
Accounts receivable	98,713	76,117
Due from related parties		1,561
Inventories, net	81,034	81,675
Deferred income taxes	26,653	28,112
Derivative financial instruments	27,461	13,785
Other assets	22,510	16,513
Total current assets	555,448	495,795
Noncurrent assets
Property, plant and equipment, net	1,675,611	1,678,294
Goodwill	35,634	35,634
Deferred income taxes	94,300	101,075
Other assets	43,562	40,525
Total assets	2,404,555	2,351,323
Current liabilities
Accounts payable	64,650	49,589
Royalties and production taxes	135,391	129,351
Accrued expenses	61,835	50,364
Due to related parties	59
Current portion of federal coal lease	63,191	63,191
Other liabilities	2,772	2,770
Total current liabilities	347,383	314,750
Noncurrent liabilities
Senior notes	596,621	596,506
Federal coal lease obligations, net of current portion	122,928	122,928
Asset retirement obligations, net of current portion	238,684	238,991
Other liabilities	53,749	50,073
Total liabilities	1,456,418	1,420,301
Commitments and Contingencies (Note 9)
Total member's equity	948,137	931,022
Total liabilities and members' equity	2,404,555	2,351,323
Parent Company (CPE Resources)
Current assets
Cash and cash equivalents	217,514	195,076	483,522	401,087
Investments in marketable securities	80,473	80,341
Inventories, net	5,501	6,741
Derivative financial instruments		138
Other assets	61	7
Total current assets	303,548	282,303
Noncurrent assets
Property, plant and equipment, net	9,489	9,239
Deferred income taxes	23,869	22,807
Other assets	1,718,877	1,682,267
Total assets	2,055,783	1,996,616
Current liabilities
Accounts payable	2,357	2,558
Accrued expenses	14,670	2,087
Due to related parties	427,123	396,137
Current deferred income taxes	27	27
Other liabilities	50	49
Total current liabilities	444,227	400,868
Noncurrent liabilities
Senior notes	596,621	596,506
Other liabilities	45	61
Total liabilities	1,040,893	997,425
Commitments and Contingencies (Note 9)
Total member's equity	1,014,890	999,190
Total liabilities and members' equity	2,055,783	1,996,615
Guarantor Subsidiaries
Current assets
Cash and cash equivalents	318		2	2
Accounts receivable	24,257	74,008
Due from related parties	482,522	385,102
Inventories, net	71,295	71,312
Deferred income taxes	19,665	21,124
Derivative financial instruments	27,461	13,647
Other assets	21,994	16,100
Total current assets	647,513	581,293
Noncurrent assets
Property, plant and equipment, net	1,662,410	1,666,020
Goodwill	35,634	35,634
Deferred income taxes	18,211	24,650
Total assets	2,363,768	2,307,597
Current liabilities
Accounts payable	60,189	45,896
Royalties and production taxes	134,101	126,726
Accrued expenses	43,536	41,529
Current portion of federal coal lease		63,191
Other liabilities	1,756	1,754
Total current liabilities	302,773	279,096
Noncurrent liabilities
Federal coal lease obligations, net of current portion	122,928	122,928
Asset retirement obligations, net of current portion	165,180	164,626
Other liabilities	72,087	77,655
Total liabilities	662,968	644,305
Commitments and Contingencies (Note 9)
Total member's equity	1,700,800	1,663,293
Total liabilities and members' equity	2,363,768	2,307,598
Non-Guarantor Subsidiaries
Current assets
Cash and cash equivalents	771	2,615	4,516	3,151
Accounts receivable	74,456	2,108
Due from related parties		42
Inventories, net	4,239	3,622
Other assets	120	117
Total current assets	79,586	8,504
Noncurrent assets
Property, plant and equipment, net	3,712	3,035
Deferred income taxes	17,280	18,679
Other assets	4,470	4,470
Total assets	105,048	34,688
Current liabilities
Accounts payable	2,041	1,117
Royalties and production taxes	1,290	2,625
Accrued expenses	2,237	292
Due to related parties	64,363
Other liabilities	966	966
Total current liabilities	70,896	5,001
Noncurrent liabilities
Asset retirement obligations, net of current portion	73,504	74,365
Other liabilities	6,090	5,806
Total liabilities	150,489	85,172
Commitments and Contingencies (Note 9)
Total member's equity	(45,441)	(50,484)
Total liabilities and members' equity	105,048	34,688
Eliminations
Current assets
Due from related parties	(482,522)	(385,144)
Deferred income taxes	(27)	(28)
Total current assets	(482,549)	(385,172)
Noncurrent assets
Other assets	(1,679,833)	1,646,259
Total assets	(2,162,382)	(2,031,430)
Current liabilities
Due to related parties	(482,522)	(385,144)
Current deferred income taxes	(27)	(27)
Total current liabilities	(482,549)	(385,172)
Noncurrent liabilities
Other liabilities	(24,475)	(33,449)
Total liabilities	(507,024)	(418,621)
Commitments and Contingencies (Note 9)
Total member's equity	(1,655,358)	(1,612,809)
Total liabilities and members' equity	(2,162,382)	(2,031,430)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Current assets
Cash and cash equivalents	218,604	197,691	488,040	404,240
Investments in marketable securities	80,473	80,341
Accounts receivable	98,713	76,117
Inventories, net	81,034	81,675
Deferred income taxes	19,638	21,096
Derivative financial instruments	27,461	13,785
Other assets	22,175	16,224
Total current assets	548,098	486,929
Noncurrent assets
Property, plant and equipment, net	1,675,611	1,678,294
Goodwill	35,634	35,634
Deferred income taxes	59,361	66,136
Other assets	43,514	40,478
Total assets	2,362,218	2,307,471
Current liabilities
Accounts payable	64,587	49,571
Royalties and production taxes	135,391	129,351
Accrued expenses	60,443	43,908
Due to related parties	8,963	10,993
Current portion of federal coal lease	63,191	63,191
Other liabilities	2,772	2,769
Total current liabilities	335,347	299,783
Noncurrent liabilities
Senior notes	596,621	596,506
Federal coal lease obligations, net of current portion	122,928	122,928
Asset retirement obligations, net of current portion	238,684	238,991
Other liabilities	53,748	50,073
Total liabilities	1,347,328	1,308,281
Commitments and Contingencies (Note 9)
Total member's equity	1,014,890	999,190
Total liabilities and members' equity	$ 2,362,218	$ 2,307,471