CPE Inc. (Parent Company Only) Financial Information (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating activities
Net income (loss)	$ 173,720	$ 189,797	$ 33,737
Net cash provided by operating activities	247,355	296,784	324,776
Investing activities
Net cash used in investing activities	(347,865)	(175,672)	(192,020)
Financing activities
Net cash used in financing activities	(106,039)	(56,973)	(60,971)
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
Cloud Peak Energy Inc.
Operating activities
Net income (loss)	173,720	189,797	33,737
Adjustments to reconcile net income (loss) to net cash used in operating activities	(173,720)	(189,798)	(33,736)
Net cash provided by operating activities		(1)	1
Financing activities
Increase (decrease) in cash and cash equivalents			1
Cash and cash equivalents at beginning of period		1
Cash and cash equivalents at end of period			$ 1