Derivatives (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended													9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 One Year from Balance Sheet Date [Member] Coal [Member]	Sep. 30, 2011 One Year from Balance Sheet Date [Member] Coal [Member] Coal Purchases [Member]	Sep. 30, 2011 More than One and within Two Years from Balance Sheet Date [Member] Coal [Member]	Sep. 30, 2011 More than One and within Two Years from Balance Sheet Date [Member] Coal [Member] Coal Purchases [Member]	Sep. 30, 2011 More than Two and within Three Years from Balance Sheet Date [Member] Coal [Member]	Sep. 30, 2011 More than Three and within Four Years from Balance Sheet Date [Member] Coal [Member]	Sep. 30, 2011 More than Four and within Five Years from Balance Sheet Date [Member] Coal [Member]	Sep. 30, 2011 Heating Oil [Member] Heating Oil-diesel purchases [Member]	Sep. 30, 2011 Current Assets [Member]	Dec. 31, 2010 Current Assets [Member]	Sep. 30, 2011 Heating oil-fuel surchages [Member]	Sep. 30, 2011 Year One [Member]	Sep. 30, 2011 Year Two [Member]	Sep. 30, 2011 Year Three [Member]	Sep. 30, 2011 Minimum Purchase [Member]	Sep. 30, 2011 Maximum Purchase [Member]
Gallons of diesel fuel purchased annually																			90,000,000	100,000,000
Percent of diesel purchases hedged																70.00%	55.00%
Quantities under derivative contracts					500,000	500,000	1,700,000	200,000	700,000	1,400,000	700,000	68,000,000			19,100,000
Value of trading portfolio realized																$ 2.4	$ (0.9)	$ (1.0)
Current asset for the right to reclaim cash collateral													10.7	10.3
Net unrealized and realized gains (losses) related to trading portfolio	(2.2)	(0.8)	1.7	(2.9)
Gain on derivative contracts designated as hedge instruments in cash flow hedges to be reclassified from OCI into earnings during the next twelve months	$ 9.3		$ 9.3