Regulatory Assets and Liabilities, Regulatory Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Regulatory Assets [Line Items]
Regulatory Asset, Current	$ 20,036,000		$ 14,866,000
Regulatory Asset, Noncurrent	280,693,000		233,193,000
Past expenditures not currently earning a return	12,100,000		100,000
Environmental Remediation Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	4,376,000		4,376,000
Regulatory Asset, Noncurrent	147,793,000		117,684,000
Regulatory asset, remaining amortization period	Various
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current	6,837,000	[1]	8,202,000	[1]
Regulatory Asset, Noncurrent	91,601,000	[1]	85,220,000	[1]
Regulatory asset, remaining amortization period	Various
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current	0		0
Regulatory Asset, Noncurrent	16,711,000		12,679,000
Regulatory asset, remaining amortization period	Plant lives
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current	801,000		801,000
Regulatory Asset, Noncurrent	4,936,000		5,737,000
Regulatory asset, remaining amortization period	Term of related debt
State Commission Adjustments
Regulatory Assets [Line Items]
Regulatory Asset, Current	488,000		410,000
Regulatory Asset, Noncurrent	11,650,000		9,355,000
Regulatory asset, remaining amortization period	Plant lives
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	0		404,000
Regulatory Asset, Noncurrent	0		0
Regulatory asset, remaining amortization period	Less than one year
Deferred Income Tax Adjustment
Regulatory Assets [Line Items]
Regulatory Asset, Current	0		0
Regulatory Asset, Noncurrent	1,514,000		1,763,000
Regulatory asset, remaining amortization period	Typically plant lives
Recoverable Purchased Natural Gas And Electric Energy Costs
Regulatory Assets [Line Items]
Regulatory Asset, Current	6,946,000		673,000
Regulatory Asset, Noncurrent	0		0
Regulatory asset, remaining amortization period	Less than one year
Monticello EPU Cost Deferral
Regulatory Assets [Line Items]
Regulatory Asset, Current	0		0
Regulatory Asset, Noncurrent	5,237,000		0
Regulatory asset, remaining amortization period	Pending rate cases
Other Regulatory Assets
Regulatory Assets [Line Items]
Regulatory Asset, Current	588,000		0
Regulatory Asset, Noncurrent	$ 1,251,000		$ 755,000
Regulatory asset, remaining amortization period	Various

[1] Includes the non-qualified pension plan.