Supplemental Guarantor/Non-Guarantor Financial Information (CPE Resources only) (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from continuing operations
Net cash provided by (used in) operating activities	$ 37,343	$ 53,009
Investing activities
Purchases of property, plant and equipment	(13,030)	(14,338)
Investments in marketable securities	(13,051)	(28,349)
Maturity and redemption of investments	12,920	5,930
Investment in project development	(2,429)
Return of restricted cash		71,245
Partnership escrow deposit		(4,470)
Other	23	773
Net cash provided by (used in) investing activities	(15,567)	30,791
Financing activities
Payment of debt issuance costs	(863)
Net cash used in financing activities	(863)
Net increase in cash and cash equivalents	20,913	83,800
Cash and cash equivalents at beginning of period	197,691	404,240
Cash and cash equivalents at end of period	218,604	488,040
Parent Company (CPE Resources)
Cash flows from continuing operations
Net cash provided by (used in) operating activities	23,654	35,665
Investing activities
Purchases of property, plant and equipment	(1,085)	(2,056)
Investments in marketable securities	(13,051)	(28,349)
Maturity and redemption of investments	12,290	5,930
Return of restricted cash		71,245
Net cash provided by (used in) investing activities	(1,216)	46,770
Financing activities
Net increase in cash and cash equivalents	22,438	82,435
Cash and cash equivalents at beginning of period	195,076	401,087
Cash and cash equivalents at end of period	217,514	483,522
Guarantor Subsidiaries
Cash flows from continuing operations
Net cash provided by (used in) operating activities	21,533	15,919
Investing activities
Purchases of property, plant and equipment	(11,946)	(12,222)
Investment in project development	(2,429)
Contributions made to subsidiary	(6,000)	(4,470)
Other	23	773
Net cash provided by (used in) investing activities	(20,352)	(15,919)
Financing activities
Payment of debt issuance costs	(863)
Net cash used in financing activities	(863)
Net increase in cash and cash equivalents	318
Cash and cash equivalents at beginning of period		2
Cash and cash equivalents at end of period	318	2
Non-Guarantor Subsidiaries
Cash flows from continuing operations
Net cash provided by (used in) operating activities	(7,844)	1,425
Investing activities
Purchases of property, plant and equipment		(60)
Partnership escrow deposit		(4,470)
Net cash provided by (used in) investing activities		(4,530)
Financing activities
Contributions received from parent	6,000	4,470
Net cash used in financing activities	6,000	4,470
Net increase in cash and cash equivalents	(1,844)	1,365
Cash and cash equivalents at beginning of period	2,615	3,151
Cash and cash equivalents at end of period	771	4,516
Eliminations
Investing activities
Contributions made to subsidiary	6,000	4,470
Net cash provided by (used in) investing activities	6,000	4,470
Financing activities
Contributions received from parent	(6,000)	(4,470)
Net cash used in financing activities	(6,000)	(4,470)
CLOUD PEAK ENERGY RESOURCES LLC AND SUBSIDIARIES
Cash flows from continuing operations
Net cash provided by (used in) operating activities	37,343	53,009
Investing activities
Purchases of property, plant and equipment	(13,030)	(14,338)
Investments in marketable securities	(13,051)	(28,349)
Maturity and redemption of investments	12,920	5,930
Investment in project development	(2,429)
Return of restricted cash		71,245
Partnership escrow deposit		(4,470)
Other	23	773
Net cash provided by (used in) investing activities	(15,567)	30,791
Financing activities
Payment of debt issuance costs	(863)
Net cash used in financing activities	(863)
Net increase in cash and cash equivalents	20,913	83,800
Cash and cash equivalents at beginning of period	197,691	404,240
Cash and cash equivalents at end of period	$ 218,604	$ 488,040