CONDENSED CONSOLIDATED BALANCE SHEETS (Cloud Peak Energy Inc. 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
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 tax agreement liability	19,485	19,485
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
Tax agreement liability, net of current portion	97,053	97,053
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
Common stock ($0.01 par value; 200,000 shares authorized; 61,113 and 61,114 shares issued and 60,822 and 60,839 outstanding at March 31, 2013 and December 31, 2012, respectively)	608	608
Treasury stock (291 shares and 276 shares at March 31, 2013 and December 31, 2012, respectively)	(5,650)	(5,390)
Additional paid-in capital	552,128	550,452
Retained earnings	421,209	405,813
Accumulated other comprehensive loss	(20,158)	(20,461)
Total equity	948,137	931,022
Total liabilities and equity	$ 2,404,555	$ 2,351,323