Tax Agreement Liability (CPE Inc. only) (Details) (USD $)	3 Months Ended					12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Tax Agreement Liability
Payments made						$ 25,097,000	$ 9,409,000	$ 1,685,000
Cloud Peak Energy Inc.
Tax Agreement Liability
Balance at the beginning of the period						170,636,000	190,110,000	54,509,000
Changes	(29,000,000)	(52,799,000)	42,733,000	123,556,000	13,731,000
Payments made						(25,098,000)	(9,409,000)	(1,685,000)
Balance at the end of the period				190,110,000		116,538,000	170,636,000	190,110,000
Less current portion				18,226,000		19,485,000	19,113,000	18,226,000
Long-term tax agreement liability				171,885,000		97,053,000	151,523,000	171,885,000
Estimated payment under tax agreement liability						19,500,000
Future average annual payment						$ 12,600,000