Derivatives (Narrative) (Details) $ in Thousands, gal in Millions	9 Months Ended	12 Months Ended
	Oct. 01, 2016 USD ($)	Dec. 31, 2017 USD ($) $ / option gal	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Right to reclaim cash collateral, asset		$ 50,042		$ 69,247
Right to reclaim cash collateral, liability		50,042		69,247
Net unrealized and realized gains (losses) related to trading portfolio	$ (900)	(2,000)	$ 5,700
Derivative contracts expected to be reclassified from OCI into earnings		(1,200)
2018
Derivative [Line Items]
Value of trading portfolio realized		$ (1,200)
Diesel purchases | Heating oil
Derivative [Line Items]
Quantities under derivative contracts | gal		26.2
Average strike price (in dollars per option) | $ / option		1.84
Diesel purchases | Minimum
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal		42.0
Diesel purchases | Maximum
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal		46.0
Other current liabilities
Derivative [Line Items]
Right to reclaim cash collateral, liability				$ 2,800
Other current assets
Derivative [Line Items]
Right to reclaim cash collateral, asset		$ 16,200