Derivatives (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Derivative [Line Items]
Current liability for the obligation to return cash collateral	$ 3.8		$ 3.8
Current asset for the right to reclaim cash collateral					2.2
Net unrealized and realized gains (losses) related to trading portfolio	1.9	0.9	5.4	3.0
Gain on derivative contracts designated as hedge instruments in cash flow hedges to be reclassified from OCI into earnings during the next twelve months	2.0		2.0
Year One, Remainder [Member]
Derivative [Line Items]
Value of trading portfolio realized			4.7
Year Two [Member]
Derivative [Line Items]
Value of trading portfolio realized			$ 4.2
Diesel Purchases [Member] | Year One, Remainder [Member]
Derivative [Line Items]
Percent of expected requirements covered	87.00%		87.00%
Diesel Purchases [Member] | Year Two [Member]
Derivative [Line Items]
Percent of expected requirements covered	83.00%		83.00%
Diesel Purchases [Member] | Heating Oil [Member]
Derivative [Line Items]
Quantities under derivative contracts	69,000,000		69,000,000
Diesel Purchases [Member] | Minimum [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually			57,000,000
Diesel Purchases [Member] | Maximum [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually			67,000,000
Heating Oil-Fuel Surcharges [Member] | Heating Oil [Member]
Derivative [Line Items]
Quantities under derivative contracts	1,300,000		1,300,000