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	[2]	$ 11,392,000	[1],[3],[4]	$ 9,687,000
Asset Retirement Obligation, Liabilities Incurred	[1],[4]	949,000		0
Asset Retirement Obligation, Liabilities Settled	[2]	0		(29,000)
Accretion		421,000	[1],[4]	342,000	[2]
Cash Flow Revisions		1,661,000	[1],[4]	1,392,000	[2]
Ending balance	[1],[4]	14,423,000		11,392,000	[2],[3]
Electric Plant Steam Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		2,194,000	[3]	2,145,000
Asset Retirement Obligation, Liabilities Incurred	[5]	949,000
Asset Retirement Obligation, Liabilities Settled				0
Accretion		50,000		49,000
Cash Flow Revisions		0		0
Ending balance		3,193,000	[4]	2,194,000	[3]
Electric Plant Steam Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		452,000	[3]	617,000
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled				0
Accretion		15,000		18,000
Cash Flow Revisions		0		(183,000)
Ending balance		467,000	[4]	452,000	[3]
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		32,000	[3]	72,000
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled				0
Accretion		3,000		3,000
Cash Flow Revisions		0		(43,000)
Ending balance		35,000	[4]	32,000	[3]
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		376,000	[3]	391,000
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled				(29,000)
Accretion		12,000		14,000
Cash Flow Revisions		0		0
Ending balance		388,000	[4]	376,000	[3]
Natural Gas Plant Gas Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		8,293,000	[3]	6,367,000
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled				0
Accretion		339,000		256,000
Cash Flow Revisions		1,661,000	[6]	1,670,000
Ending balance		10,293,000	[4]	8,293,000	[3]
Common and Other Property Common Miscellaneous
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance		45,000	[3]	95,000
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled				0
Accretion		2,000		2,000
Cash Flow Revisions		0		(52,000)
Ending balance		$ 47,000	[4]	$ 45,000	[3]

[1] Included in other long-term liabilities balance in the consolidated balance sheet.

[2] Included in other long-term liabilities balance in the consolidated balance sheet.

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

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

[5] The liability recognized relates to asbestos at the French Island plant.

[6] Changes in the gas distribution ARO are mainly related to increased labor costs.