Rate and Regulatory Matters (Schedule of Regulatory Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory Assets [Line Items]
Regulatory Assets	$ 118,144	$ 110,479
Retirement Benefits [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 45,419	$ 44,281
Remaining recovery period	Various	Various
Income Taxes [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 66,759	$ 60,151
Remaining recovery period	Various	Various
Rate Cases, Tank Painting, and Other [Member]
Regulatory Assets [Line Items]
Regulatory Assets	$ 5,966	$ 6,047
Remaining recovery period	2-10 years	2-10 years