Regulatory Assets and Liabilities, Regulatory Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory asset, current	$ 12,048,000		$ 14,133,000
Regulatory assets, non-current	240,459,000		229,910,000
Past expenditures not currently earning a return	400,000
Environmental Remediation Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various
Regulatory asset, current	2,521,000		0
Regulatory assets, non-current	109,162,000		108,181,000
Pension and Retiree Medical Obligations [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various	[1]
Regulatory asset, current	7,328,000	[1]	8,197,000	[1]
Regulatory assets, non-current	104,426,000	[1]	98,045,000	[1]
Recoverable Deferred Taxes on AFUDC Recorded in Plant [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Plant lives
Regulatory asset, current	0		0
Regulatory assets, non-current	10,458,000		9,630,000
Losses on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Term of related debt
Regulatory asset, current	800,000		843,000
Regulatory assets, non-current	6,538,000		7,338,000
State Commission Adjustments [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Plant lives
Regulatory asset, current	339,000		277,000
Regulatory assets, non-current	7,533,000		4,506,000
Contract Valuation Adjustments [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Term of related contract	[2]
Regulatory asset, current	11,000	[2]	2,515,000	[2]
Regulatory assets, non-current	0	[2]	0	[2]
Conservation Programs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Less than one year
Regulatory asset, current	691,000		2,301,000
Regulatory assets, non-current	0		0
Deferred Income Tax Adjustment [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Typically plant lives
Regulatory asset, current	0		0
Regulatory assets, non-current	1,974,000		1,868,000
Recoverable Purchased Natural Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Less than one year
Regulatory asset, current	358,000		0
Regulatory assets, non-current	0		0
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various
Regulatory asset, current	0		0
Regulatory assets, non-current	$ 368,000		$ 342,000

[1] Includes the non-qualified pension plan.

[2] Includes valuation adjustments on natural gas commodity purchases.