Basis of Presentation (Details)	Dec. 31, 2012	Dec. 05, 2012
Equity details
Ownership interest in Decker (as a percent)	50.00%	50.00%

The percentage of ownership of common stock or equity participation in the investee accounted for under the proportional consolidation method of accounting.

No definition available.

Name:	cld_ProportionalConsolidationMethodInvestmentOwnershipPercentage
Namespace Prefix:	cld_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant