Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 28,663,000	[1]	$ 27,233,000
Liabilities Incurred	0		0
Liabilities settled	0		0
Accretion	1,537,000		1,451,000
Cash flow revisions	(1,676,000)	[2]	(21,000)
Ending balance	28,524,000	[3]	28,663,000	[1]
Electric Plant Steam Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	19,070,000	[1]	17,981,000
Accretion	1,155,000		1,089,000
Cash flow revisions	(1,676,000)	[2]	0
Ending balance	18,549,000	[3]	19,070,000	[1]
Electric Plant Steam Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,593,000	[1]	1,513,000
Accretion	85,000		80,000
Cash flow revisions	0	[2]	0
Ending balance	1,678,000	[3]	1,593,000	[1]
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	6,799,000	[1]	6,559,000
Accretion	249,000		240,000
Cash flow revisions	0	[2]	0
Ending balance	7,048,000	[3]	6,799,000	[1]
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,201,000	[1]	1,180,000
Accretion	48,000		42,000
Cash flow revisions	0	[2]	(21,000)
Ending balance	$ 1,249,000	[3]	$ 1,201,000	[1]

[1] There were no ARO liabilities recognized or settled during the year ended Dec. 31, 2016.

[2] In 2017, an asbestos ARO was revised for changes in timing of estimated cash flows.

[3] There were no ARO liabilities recognized or settled during the year ended Dec. 31, 2017.