Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	$ 329,996	$ 343,183	$ 668,048	$ 716,086
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	281,603	266,073	557,631	549,018
Depreciation and depletion	25,459	22,285	48,671	45,675
Accretion	4,126	3,422	8,253	6,070
Derivative mark-to-market gains	(12,284)	(20,183)	(25,936)	(18,127)
Selling, general and administrative expenses	12,834	12,556	26,442	27,298
Other operating costs	1,191	308	1,301	401
Total costs and expenses	312,929	284,461	616,362	610,335
Operating income (loss)	17,067	58,722	51,686	105,751
Other income (expense)
Interest income	63	312	188	758
Interest expense	(10,315)	(7,936)	(20,799)	(13,786)
Other, net	43	(111)	(198)	(53)
Total other expense	(10,209)	(7,735)	(20,809)	(13,081)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	6,858	50,987	30,877	92,670
Income tax benefit (expense)	(2,462)	(18,806)	(11,297)	(33,908)
Earnings from unconsolidated affiliates, net of tax	313	1,497	524	1,534
Net income	4,709	33,678	20,104	60,296
Other comprehensive income
Retiree medical plan amortization of prior service cost	444	394	888	788
Other postretirement plan adjustments			30	90
Income tax on retiree medical plan and pension adjustments	(160)	(142)	(330)	(316)
Other comprehensive income	284	252	588	562
Total comprehensive income	4,993	33,930	20,692	60,858
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)	10	(3)	19	3
Depreciation and depletion	627	463	1,280	1,020
Selling, general and administrative expenses	155	205	420	419
Other operating costs	555		554
Total costs and expenses	1,347	665	2,273	1,442
Operating income (loss)	(1,347)	(665)	(2,272)	(1,442)
Other income (expense)
Interest income	63	312	188	758
Interest expense	(9,975)	(7,397)	(20,123)	(13,062)
Other, net	(165)		(323)
Total other expense	(10,077)	(7,085)	(20,258)	(12,304)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	(11,424)	(7,750)	(22,530)	(13,746)
Income tax benefit (expense)	3,204	7,702	4,205	14,852
Earnings from unconsolidated affiliates, net of tax	5	6	9	12
Earnings (losses) from consolidated affiliates, net of tax	12,924	33,720	38,420	59,178
Net income	4,709	33,678	20,104	60,296
Other comprehensive income
Retiree medical plan amortization of prior service cost	444	394	888	788
Other postretirement plan adjustments			30	90
Income tax on retiree medical plan and pension adjustments	(160)	(142)	(330)	(316)
Other comprehensive income	284	252	588	562
Total comprehensive income	4,993	33,930	20,692	60,858
Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	324,658	337,085	660,223	706,313
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	275,211	258,580	547,666	535,398
Depreciation and depletion	23,953	21,101	47,201	42,745
Accretion	2,924	2,330	5,862	4,660
Derivative mark-to-market gains	(12,284)	(20,183)	(25,936)	(18,127)
Selling, general and administrative expenses	12,679	12,350	26,022	26,878
Other operating costs	636	308	747	401
Total costs and expenses	303,119	274,486	601,562	591,955
Operating income (loss)	21,539	62,599	58,661	114,358
Other income (expense)
Interest expense	(238)	(524)	(514)	(692)
Other, net	43	(110)	(240)	(53)
Total other expense	(195)	(634)	(754)	(745)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	21,344	61,965	57,907	113,613
Income tax benefit (expense)	(6,791)	(27,654)	(17,108)	(51,320)
Earnings from unconsolidated affiliates, net of tax	308	1,491	515	1,522
Earnings (losses) from consolidated affiliates, net of tax	(1,937)	(2,082)	(2,894)	(4,638)
Net income	12,924	33,720	38,420	59,177
Other comprehensive income
Retiree medical plan amortization of prior service cost	444	394	888	788
Other postretirement plan adjustments			30	90
Income tax on retiree medical plan and pension adjustments	(160)	(142)	(330)	(316)
Other comprehensive income	284	252	588	562
Total comprehensive income	13,208	33,972	39,008	59,739
Non-Guarantor Subsidiaries
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	5,338	6,098	7,824	9,773
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	6,382	7,497	9,947	13,617
Depreciation and depletion	878	720	190	1,910
Accretion	1,202	1,092	2,391	1,411
Total costs and expenses	8,462	9,309	12,528	16,938
Operating income (loss)	(3,124)	(3,211)	(4,704)	(7,165)
Other income (expense)
Interest expense	(102)	(16)	(162)	(32)
Other, net	165		365
Total other expense	63	(16)	203	(32)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	(3,061)	(3,227)	(4,501)	(7,198)
Income tax benefit (expense)	1,124	1,145	1,607	2,560
Net income	(1,937)	(2,082)	(2,894)	(4,638)
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	(1,937)	(2,082)	(2,894)	(4,580)
Eliminations
Other income (expense)
Earnings (losses) from consolidated affiliates, net of tax	(10,987)	(31,638)	(35,526)	(54,540)
Net income	(10,987)	(31,638)	(35,526)	(54,539)
Other comprehensive income
Retiree medical plan amortization of prior service cost	(444)	(394)	(888)	(788)
Other postretirement plan adjustments			(30)	(180)
Income tax on retiree medical plan and pension adjustments	160	142	330	348
Other comprehensive income	(284)	(252)	(588)	(620)
Total comprehensive income	(11,271)	(31,890)	(36,114)	(55,159)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Supplemental Condensed Consolidating Statement of Operations and Comprehensive Income
Revenue	329,996	343,183	668,048	716,086
Costs and expenses
Cost of product sold (exclusive of depreciation, depletion, amortization and accretion, shown separately)	281,603	266,073	557,631	549,018
Depreciation and depletion	25,459	22,285	48,671	45,675
Accretion	4,126	3,422	8,253	6,070
Derivative mark-to-market gains	(12,284)	(20,183)	(25,936)	(18,127)
Selling, general and administrative expenses	12,834	12,556	26,442	27,298
Other operating costs	1,191	308	1,301	401
Total costs and expenses	312,929	284,461	616,362	610,335
Operating income (loss)	17,067	58,722	51,686	105,751
Other income (expense)
Interest income	63	312	188	758
Interest expense	(10,315)	(7,936)	(20,799)	(13,786)
Other, net	43	(111)	(198)	(53)
Total other expense	(10,209)	(7,735)	(20,809)	(13,081)
Income (loss) before income tax provision and earnings (losses) from unconsolidated affiliates	6,858	50,987	30,877	92,670
Income tax benefit (expense)	(2,462)	(18,806)	(11,297)	(33,908)
Earnings from unconsolidated affiliates, net of tax	313	1,497	524	1,534
Net income	4,709	33,678	20,104	60,296
Other comprehensive income
Retiree medical plan amortization of prior service cost	444	394	888	788
Other postretirement plan adjustments			30	90
Income tax on retiree medical plan and pension adjustments	(160)	(142)	(330)	(316)
Other comprehensive income	284	252	588	562
Total comprehensive income	$ 4,993	$ 33,930	$ 20,692	$ 60,858