Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	$ 27,233,000	[1]	$ 26,031,000
Liabilities Incurred	0		0
Liabilities settled	0		0
Accretion	1,451,000		1,383,000
Cash flow revisions	(21,000)		(181,000)
Ending balance	28,663,000	[2]	27,233,000	[1]
Electric Plant Steam Production Asbestos
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	17,981,000	[1]	16,957,000
Liabilities Incurred	0		0
Liabilities settled	0		0
Accretion	1,089,000		1,024,000
Cash flow revisions	0		0
Ending balance	19,070,000	[2]	17,981,000	[1]
Electric Plant Steam Production Ash Containment
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,513,000	[1]	1,609,000
Liabilities Incurred	0		0
Liabilities settled	0		0
Accretion	80,000		85,000
Cash flow revisions	0		(181,000)
Ending balance	1,593,000	[2]	1,513,000	[1]
Electric Plant Electric Distribution
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	6,559,000	[1]	6,327,000
Liabilities Incurred	0		0
Liabilities settled	0		0
Accretion	240,000		232,000
Cash flow revisions	0		0
Ending balance	6,799,000	[2]	6,559,000	[1]
Electric Plant Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,180,000	[1]	1,138,000
Liabilities Incurred	0		0
Liabilities settled	0		0
Accretion	42,000		42,000
Cash flow revisions	(21,000)		0
Ending balance	$ 1,201,000	[2]	$ 1,180,000	[1]

[1] (a) There were no ARO liabilities recognized or settled during the year ended Dec. 31, 2015.

[2] (a) There were no ARO liabilities recognized or settled during the year ended Dec. 31, 2016.