Contingencies (Narrative) (Details) (USD $) In Millions, unless otherwise specified					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Aug. 25, 2011	Aug. 13, 2007	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Minimum [Member]
Purchase replacement coal				$ 100
Aggregate amount of past and future damages					377.0	228.0
Damages and interest awarded			104.1
Appeal Bond	124.9
Amount accrued	32.8	117.2
Amount accrued current portion	4.4	6.3
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	216.2
Unrecorded Unconditional Purchase Obligation, Due within Two Years	135.3
Unrecorded Unconditional Purchase Obligation, Due within Three Years	135.1
Unrecorded Unconditional Purchase Obligation, Due within Four Years	102.5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	98.6
Unrecorded Unconditional Purchase Obligation, Due after Five Years	$ 445.6