Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	$ 338,052	$ 372,903
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization 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	(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 (loss)	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 (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	24,020	41,681
Income tax benefit (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 cost	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
Parent Company (CPE Resources)
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	1
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	8	7
Depreciation and depletion	652	556
Selling, general and administrative expenses	265	214
Total costs and expenses	925	777
Operating income (loss)	(924)	(777)
Other income (expense)
Interest income	125	446
Interest expense	(10,147)	(5,666)
Other, net	(158)
Total other expense	(10,180)	(5,220)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	(11,105)	(5,996)
Income tax benefit (expense)	1,000	7,151
Earnings from unconsolidated affiliates, net of tax	4	6
Earnings (losses) from consolidated affiliates, net of tax	25,496	25,457
Net income	15,395	26,618
Other comprehensive income
Retiree medical plan amortization of prior service cost	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
Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	335,565	369,228
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	272,455	276,817
Depreciation and depletion	23,249	21,645
Accretion	2,938	2,330
Derivative mark-to-market gains	(13,652)	2,056
Selling, general and administrative expenses	13,342	14,529
Other operating costs	110	93
Total costs and expenses	298,442	317,469
Operating income (loss)	37,123	51,759
Other income (expense)
Interest expense	(276)	(168)
Other, net	(283)	58
Total other expense	(559)	(110)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	36,564	51,648
Income tax benefit (expense)	(10,317)	(23,666)
Earnings from unconsolidated affiliates, net of tax	207	32
Earnings (losses) from consolidated affiliates, net of tax	(958)	(2,557)
Net income	25,496	25,457
Other comprehensive income
Retiree medical plan amortization of prior service cost	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	25,799	25,767
Non-Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	2,486	3,675
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	3,566	6,121
Depreciation and depletion		1,190
Depreciation and depletion	(689)
Accretion	1,189	319
Total costs and expenses	4,066	7,630
Operating income (loss)	(1,580)	(3,955)
Other income (expense)
Interest expense	(60)	(16)
Other, net	200
Total other expense	140	(16)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	(1,439)	(3,971)
Income tax benefit (expense)	482	1,414
Net income	(958)	(2,557)
Other comprehensive income
Other postretirement plan adjustments		90
Income tax on retiree medical plan and pension adjustments		(32)
Other comprehensive income		58
Total comprehensive income	(958)	(2,499)
Eliminations
Other income (expense)
Earnings (losses) from consolidated affiliates, net of tax	(24,538)	(22,900)
Net income	(24,538)	(22,900)
Other comprehensive income
Retiree medical plan amortization of prior service cost	(444)	(394)
Other postretirement plan adjustments	(30)	(180)
Income tax on retiree medical plan and pension adjustments	171	206
Other comprehensive income	(303)	(368)
Total comprehensive income	(24,841)	(23,268)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	338,052	372,903
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization 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	(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 (loss)	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 (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	24,020	41,681
Income tax benefit (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 cost	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