Derivatives (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Derivative [Line Items]
Current asset (liability) for the right to reclaim cash collateral	$ (4,900,000)		$ (4,900,000)		$ 16,200,000
Net unrealized and realized gains (losses) related to trading portfolio	900,000	5,500,000	3,000,000	6,200,000
Gain on derivative contracts designated as hedge instruments in cash flow hedges to be reclassified from OCI into earnings during the next twelve months	2,900,000		2,900,000
Net derivative assets as classified in the balance sheet	27,061,000		27,061,000		32,315,000
Year One [Member]
Derivative [Line Items]
Value of trading portfolio realized			400,000
Year Two [Member]
Derivative [Line Items]
Value of trading portfolio realized			$ 7,000,000
Diesel Purchases [Member] | Year One [Member]
Derivative [Line Items]
Percent of expected requirements covered	94.00%		94.00%
Diesel Purchases [Member] | Year Two [Member]
Derivative [Line Items]
Percent of expected requirements covered	82.00%		82.00%
Diesel Purchases [Member] | Heating Oil [Member]
Derivative [Line Items]
Quantities under derivative contracts	65,000,000		65,000,000
Diesel Purchases [Member] | Minimum Annual Purchase [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
Diesel Usage Fuel Surcharges [Member] | Heating Oil [Member] | Year One [Member]
Derivative [Line Items]
Quantities under derivative contracts	2,800,000		2,800,000
Diesel Usage Fuel Surcharges [Member] | Heating Oil [Member] | Year Two [Member]
Derivative [Line Items]
Quantities under derivative contracts	4,000,000		4,000,000