Rate and Regulatory Matters (Schedule of Regulatory Assets) (Details) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2016

Dec. 31, 2015

Regulatory Assets [Line Items]

Regulatory Assets

$ 60,894

$ 58,552

Retirement Benefits [Member]

Regulatory Assets [Line Items]

Regulatory Assets

$ 40,603

$ 39,378

Remaining recovery period

Various

Various

Income Taxes [Member]

Regulatory Assets [Line Items]

Regulatory Assets

$ 15,899

$ 15,541

Remaining recovery period

Various

Various

Rate Cases, Tank Painting, and Other [Member]

Regulatory Assets [Line Items]

Regulatory Assets

$ 4,392

$ 3,633

Remaining recovery period

2-10 years

2-10 years