Commitments and Contingencies, Environmental Contingencies (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013 Federal Clean Water Act Regulation	Dec. 31, 2010 Regional Haze Rules Boiler Kiln Group	Sep. 30, 2013 Capital Addition Purchase Commitments Regional Haze Rules
Environmental Requirements [Abstract]
Number of potential regulatory options under the proposed Effluent Limitations Guidelines rule	4
Liability for estimated cost to comply with regulation			$ 343.0
Number of environmental groups who petitioned the U.S. Department of the Interior		2
Number of coal-fired boilers in Colorado		12
Number of coal-fired cement kilns in Colorado		1