Derivatives (Narrative) (Details) $ in Thousands, gal in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) $ / option gal	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Cash collateral, liability	$ 25,759	$ 17,801
Right to reclaim cash collateral, asset	25,759	17,801
Derivative contracts expected to be reclassified from OCI into earnings	400
2019
Derivative [Line Items]
Value of trading portfolio realized	$ (500)
Diesel purchases | Heating oil
Derivative [Line Items]
Quantities under derivative contracts | gal	10.1
Average strike price (in dollars per option) | $ / option	2.28
Diesel purchases | Minimum
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal	34.0
Diesel purchases | Maximum
Derivative [Line Items]
Gallons of diesel fuel purchased annually | gal	40.0
Other Current Liabilities
Derivative [Line Items]
Cash collateral, liability	$ 4,400
Other current assets
Derivative [Line Items]
Right to reclaim cash collateral, asset		$ 24,700