CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (Cloud Peak Energy Inc. and Subsidiaries) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME
Revenue	$ 338,052	$ 372,903
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, and accretion, shown separately)	276,029	282,945
Depreciation and depletion	23,212	23,391
Accretion	4,127	2,649
Derivative mark-to-market (gains) losses	(13,652)	2,056
Selling, general and administrative expenses	13,607	14,742
Other operating costs	110	93
Total costs and expenses	303,433	325,876
Operating income	34,619	47,027
Other income (expense)
Interest income	125	446
Interest expense	(10,483)	(5,850)
Other, net	(241)	58
Total other expense	(10,599)	(5,346)
Income before income tax provision and earnings from unconsolidated affiliates	24,020	41,681
Income tax expense	(8,836)	(15,101)
Earnings from unconsolidated affiliates, net of tax	211	38
Net income	15,395	26,618
Other comprehensive income
Retiree medical plan amortization of prior service costs	444	394
Other postretirement plan adjustments	30	90
Income tax on retiree medical plan and pension adjustments	(171)	(174)
Other comprehensive income	303	310
Total comprehensive income	$ 15,698	$ 26,928
Income (loss) per common share:
Basic (in dollars per share)	$ 0.25	$ 0.44
Diluted (in dollars per share)	$ 0.25	$ 0.44
Weighted-average shares outstanding - basic (in shares)	60,609	60,008
Weighted-average shares outstanding - diluted (in shares)	61,093	60,761