Regulatory Assets and Liabilities, Regulatory Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Regulatory Assets [Line Items]
Regulatory asset, current	$ 24,020,000		$ 25,244,000
Regulatory asset, noncurrent	324,081,000		294,813,000
Past expenditures not currently earning a return	25,000,000
Pension and Retiree Medical Obligations [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various	[1]
Regulatory asset, current	15,038,000	[1]	14,879,000	[1]
Regulatory asset, noncurrent	238,398,000	[1]	214,111,000	[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 asset, noncurrent	27,591,000		25,053,000
Net AROs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Plant lives	[2]
Regulatory asset, current	0	[2]	0	[2]
Regulatory asset, noncurrent	21,714,000	[2]	19,920,000	[2]
Conservation Programs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1 year	[3]
Regulatory asset, remaining amortization period, maximum	6 years	[3]
Regulatory asset, current	1,735,000	[3]	1,834,000	[3]
Regulatory asset, noncurrent	9,095,000	[3]	10,786,000	[3]
Renewable Resources and Environmental Initiatives [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period, minimum	1 year
Regulatory asset, remaining amortization period, maximum	4 years
Regulatory asset, current	4,094,000		1,458,000
Regulatory asset, noncurrent	15,101,000		10,795,000
Losses on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Term of related debt
Regulatory asset, current	1,225,000		1,228,000
Regulatory asset, noncurrent	4,922,000		6,146,000
Deferred Income Tax Adjustment [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Typically plant lives
Regulatory asset, current	0		0
Regulatory asset, noncurrent	4,859,000		6,040,000
Recoverable Electric Energy Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Less than one year
Regulatory asset, current	873,000		1,439,000
Regulatory asset, noncurrent	0		0
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Regulatory asset, remaining amortization period	Various
Regulatory asset, current	1,055,000		4,406,000
Regulatory asset, noncurrent	$ 2,401,000		$ 1,962,000

[1] Includes the non-qualified pension plan.

[2] Includes amounts recorded for future recovery of AROs.

[3] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.