CONSOLIDATED STATEMENTS OF CASH FLOWS (Cloud Peak Energy Inc. and Subsidiaries) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities
Net income	$ 173,720	$ 189,797	$ 117,197
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion, and amortization	94,575	87,127	103,220
Accretion	13,189	12,469	12,499
Earnings from unconsolidated affiliates	(1,556)	(1,801)	(3,189)
Distributions of income from unconsolidated affiliates	1,023	5,250	35
Deferred income taxes	42,210	(11,224)	28,503
Tax agreement (benefit) expense	(29,000)	19,854	19,669
Stock compensation expense	11,796	8,796	7,234
Mark-to-market gains	(22,754)	(2,275)
Other	11,795	11,506	5,377
Changes in operating assets and liabilities:
Accounts receivable	18,632	(30,074)	17,636
Inventories, net	(9,077)	(6,452)	(638)
Due to or from related parties	(1,090)	(37)	7,906
Other assets	(4,486)	4,436	(10,090)
Accounts payable and accrued expenses	(32,137)	26,327	27,040
Tax agreement liability	(25,097)	(9,409)	(1,685)
Asset retirement obligations	(5,632)	(7,506)	(5,938)
Settlement of derivatives	11,244
Net cash provided by operating activities	247,355	296,784	324,776
Investing activities
Acquisitions of Youngs Creek and CX Ranch coal and land assets	(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)
Deposit of restricted cash			(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)
Proceeds from issuance of common stock	65
Distributions to former parent			(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
Supplemental cash flow disclosures:
Interest paid	84,201	62,792	69,317
Income taxes paid, net	27,017	6,161	9,120
Supplemental noncash investing and financing activities:
Obligations to acquire federal coal leases and other mineral rights		224,658
Noncash interest capitalized	7,845	16,092	6,896
Capital expenditures included in accounts payable	$ 4,579	$ 10,893	$ 37,541