CONDENSED CONSOLIDATED BALANCE SHEETS (Cloud Peak Energy Inc. and Subsidiaries) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current assets
Cash and cash equivalents	$ 199,747	$ 197,691
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
Current portion of tax agreement liability	19,485	19,485
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
Tax agreement liability, net of current portion	97,053	97,053
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)
Equity
Common stock ($0.01 par value; 200,000 shares authorized; 61,225 and 61,114 shares issued and 60,836 and 60,839 outstanding at June 30, 2013 and December 31, 2012, respectively)	608	608
Treasury stock (389 shares and 276 shares at June 30, 2013 and December 31, 2012, respectively)	(5,650)	(5,390)
Additional paid-in capital	554,137	550,452
Retained earnings	425,917	405,813
Accumulated other comprehensive loss	(19,873)	(20,461)
Total equity	955,139	931,022
Total liabilities and equity	$ 2,401,689	$ 2,351,323