Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from continuing operations
Net cash provided by (used in) operating activities	$ 247,355	$ 296,784	$ 324,776
Investing activities
Acquisition of Youngs Creek project	(300,377)
Purchases of property, plant and equipment	(53,550)	(108,733)	(65,041)
Cash paid for capitalized interest	(50,119)	(33,989)	(26,598)
Investments in marketable securities	(67,576)	(75,228)
Maturity and redemption of investments	62,463
Investment in development projects	(7,389)
Initial payment on federal coal leases		(69,407)
Return of restricted cash	71,244	110,972	116,533
Partnership escrow deposit	(4,470)
Restricted cash deposit			(218,425)
Other	1,909	713	1,511
Net cash used in investing activities	(347,865)	(175,672)	(192,020)
Financing activities
Principal payments on federal coal leases	(102,198)	(54,630)	(50,768)
Contributions received from parent	65
Distributions			(10,203)
Other	(3,906)	(2,343)
Net cash used in financing activities	(106,039)	(56,973)	(60,971)
Net increase (decrease) in cash and cash equivalents	(206,549)	64,139	71,785
Cash and cash equivalents at beginning of period	404,240	340,101	268,316
Cash and cash equivalents at end of period	197,691	404,240	340,101
Parent Company (CPE Resources)
Cash flows from continuing operations
Net cash provided by (used in) operating activities	36,458	55,508	202,843
Investing activities
Purchases of property, plant and equipment	(8,175)	(9,721)	(4,282)
Investments in marketable securities	(67,576)	(75,228)
Maturity and redemption of investments	62,463
Return of restricted cash	71,244	110,972	116,533
Restricted cash deposit			(218,425)
Contributions made to subsidiary	(300,377)
Other	(47)
Net cash used in investing activities	(242,468)	26,023	(106,174)
Financing activities
Distributions		(162)	(21,129)
Other		(2,292)
Net cash used in financing activities		(2,454)	(21,129)
Net increase (decrease) in cash and cash equivalents	(206,010)	79,077	75,540
Cash and cash equivalents at beginning of period	401,087	322,010	246,470
Cash and cash equivalents at end of period	195,076	401,087	322,010
Guarantor Subsidiaries
Cash flows from continuing operations
Net cash provided by (used in) operating activities	219,458	247,529	136,543
Investing activities
Acquisition of Youngs Creek project	(300,377)
Purchases of property, plant and equipment	(45,185)	(98,804)	(60,610)
Cash paid for capitalized interest	(50,119)	(33,989)	(26,598)
Investment in development projects	(7,389)
Initial payment on federal coal leases		(69,407)
Contributions made to subsidiary	(12,570)	8,637
Other	1,907	713	1,437
Net cash used in investing activities	(413,733)	(192,850)	(85,771)
Financing activities
Principal payments on federal coal leases	(102,198)	(54,630)	(50,768)
Contributions received from parent	300,377
Other	(3,906)	(51)
Net cash used in financing activities	194,273	(54,681)	(50,768)
Net increase (decrease) in cash and cash equivalents	(2)	(2)	4
Cash and cash equivalents at beginning of period	2	4
Cash and cash equivalents at end of period		2	4
Non-Guarantor Subsidiaries
Cash flows from continuing operations
Net cash provided by (used in) operating activities	(8,447)	(6,090)	(3,685)
Investing activities
Purchases of property, plant and equipment	(190)	(208)	(149)
Partnership escrow deposit	(4,470)
Other			74
Net cash used in investing activities	(4,660)	(208)	(75)
Financing activities
Contributions received from parent	12,570
Distributions		(8,637)
Net cash used in financing activities	12,570	(8,637)
Net increase (decrease) in cash and cash equivalents	(537)	(14,935)	(3,760)
Cash and cash equivalents at beginning of period	3,151	18,086	21,846
Cash and cash equivalents at end of period	2,615	3,151	18,086
Eliminations
Investing activities
Contributions made to subsidiary	312,947	(8,637)
Net cash used in investing activities	312,947	(8,637)
Financing activities
Contributions received from parent	(312,947)
Distributions		8,637
Net cash used in financing activities	(312,947)	8,637
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Cash flows from continuing operations
Net cash provided by (used in) operating activities	247,469	296,947	335,701
Investing activities
Acquisition of Youngs Creek project	(300,377)
Purchases of property, plant and equipment	(53,550)	(108,733)	(65,041)
Cash paid for capitalized interest	(50,119)	(33,989)	(26,598)
Investments in marketable securities	(67,576)	(75,228)
Maturity and redemption of investments	62,463
Investment in development projects	(7,389)
Initial payment on federal coal leases		(69,407)
Return of restricted cash	71,244	110,972	116,533
Partnership escrow deposit	(4,470)
Restricted cash deposit			(218,425)
Other	1,860	713	1,511
Net cash used in investing activities	(347,914)	(175,672)	(192,020)
Financing activities
Principal payments on federal coal leases	(102,198)	(54,630)	(50,768)
Distributions		(162)	(21,129)
Other	(3,906)	(2,343)
Net cash used in financing activities	(106,104)	(57,135)	(71,897)
Net increase (decrease) in cash and cash equivalents	(206,549)	64,140	71,784
Cash and cash equivalents at beginning of period	404,240	340,100	268,316
Cash and cash equivalents at end of period	$ 197,691	$ 404,240	$ 340,100