Commitments and Contingencies, Asset Retirement Obligations (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	$ 0	$ 0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	28,500,000	28,700,000
Liabilities Incurred	0	0
Accretion	1,600,000	1,500,000
Cash flow revisions	2,300,000	(1,700,000)
Ending balance	32,400,000	28,500,000
Electric Plant Steam Production Asbestos
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	0	0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	20,300,000	20,700,000
Liabilities Incurred	0	0
Accretion	1,200,000	1,300,000
Cash flow revisions	500,000	(1,700,000)
Ending balance	22,000,000	20,300,000
Electric Plant Electric Distribution
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	0	0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	7,000,000	6,800,000
Liabilities Incurred	0	0
Accretion	300,000	200,000
Cash flow revisions	1,800,000	0
Ending balance	9,100,000	7,000,000
Electric Plant Other
Asset Retirement Obligations [Line Items]
Asset Retirement Obligation, Liabilities Settled	0	0
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Beginning balance	1,200,000	1,200,000
Liabilities Incurred	0	0
Accretion	100,000	0
Cash flow revisions	0	0
Ending balance	$ 1,300,000	$ 1,200,000