CONDENSED CONSOLIDATED BALANCE SHEETS (Cloud Peak Energy Resources LLC and Subsidiaries) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets
Cash and cash equivalents	$ 218,604	$ 197,691
Investments in marketable securities	80,473	80,341
Accounts receivable	98,713	76,117
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 obligations	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)
Equity
Accumulated other comprehensive loss	(20,158)	(20,461)
Total member's equity	948,137	931,022
Total liabilities and equity	2,404,555	2,351,323
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Current assets
Cash and cash equivalents	218,604	197,691
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 obligations	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)
Equity
Member's equity	1,035,048	1,019,651
Accumulated other comprehensive loss	(20,158)	(20,461)
Total member's equity	1,014,890	999,190
Total liabilities and equity	$ 2,362,218	$ 2,307,471