Derivatives (Narrative) (Details) $ in Thousands, gal in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) gal $ / option	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Derivative [Line Items]
Right to reclaim cash collateral, asset	$ 107,028	$ 98,686
Right to reclaim cash collateral, liability	107,028	98,686
Net unrealized and realized gains (losses) related to trading portfolio	5,700	3,200	$ 4,900
Year One [Member]
Derivative [Line Items]
Value of trading portfolio realized	$ 5,800
Diesel Purchases [Member] | Heating Oil [Member]
Derivative [Line Items]
Quantities under derivative contracts | gal	56
Average strike price (in dollars per option) | $ / option	1.98
Diesel Purchases [Member] | Minimum [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal	50
Diesel Purchases [Member] | Maximum [Member]
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal	58
Other Current Liabilities [Member]
Derivative [Line Items]
Right to reclaim cash collateral, liability	$ 2,400	2,400
Other Current Assets [Member]
Derivative [Line Items]
Right to reclaim cash collateral, asset	$ 1,700	$ 1,700