Commitments and Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Aug. 25, 2011	Aug. 13, 2007
Purchase replacement coal				$ 100
Damages and interest awarded			104.1
Payments for Legal Settlements	15.6
Amount accrued	21.2	32.8
Amount accrued current portion	7.7	4.4
Maximum [Member]
Aggregate amount of past and future damages	377.0
Minimum [Member]
Aggregate amount of past and future damages	228.0
past damages [Member]
Damages and interest awarded			13.8
future damages [Member]
Damages and interest awarded			$ 90.3