Derivatives (Narrative) (Details) $ in Thousands, gal in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) $ / option gal	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Right to reclaim cash collateral, asset	$ 17,801	$ 50,042
Derivative contracts expected to be reclassified from OCI into earnings	1,100
2019
Derivative [Line Items]
Value of trading portfolio realized	$ (200)
Diesel purchases | Heating oil
Derivative [Line Items]
Quantities under derivative contracts | gal	26.2
Average strike price (in dollars per option) | $ / option	2.30
Diesel purchases | Minimum
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal	40.0
Diesel purchases | Maximum
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal	46.0
Other current assets
Derivative [Line Items]
Right to reclaim cash collateral, asset	$ 24,700	$ 16,200