CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Cloud Peak Energy Inc. and Subsidiaries) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities
Net income	$ 15,395	$ 26,618
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and depletion	23,212	23,391
Accretion	4,127	2,649
Earnings from unconsolidated affiliates	(211)	(38)
Deferred income taxes	7,945	11,854
Stock compensation expense	1,676	3,232
Derivative mark-to-market (gains) losses	(13,652)	2,056
Other	3,318	2,229
Changes in operating assets and liabilities:
Accounts receivable	(22,574)	2,220
Inventories, net	630	(3,731)
Due to or from related parties	1,620	(232)
Other assets	(6,369)	(14,725)
Accounts payable and accrued expenses	22,577	(1,572)
Asset retirement obligations	(327)	(1,466)
Cash (paid) received for financial derivative instruments	(24)	524
Net cash provided by 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 deferred financing fees	(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
Supplemental cash flow disclosures:
Interest paid	640	653
Income taxes paid	5,377	12,638
Supplemental noncash investing and financing activities:
Non-cash interest capitalized	7,828	14,520
Capital expenditures included in accounts payable	$ 4,655	$ 3,416