Regulatory Assets and Liabilities, Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Regulatory Liabilities [Line Items]
Regulatory liability, current	$ 75,891		$ 57,104
Regulatory liability, noncurrent	91,815		105,335
Plant Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Plant lives
Regulatory liability, current	0		0
Regulatory liability, noncurrent	66,575		74,402
Deferred Electric Electric Energy Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory liability, current	61,874		43,295
Regulatory liability, noncurrent	0		0
Contract Valuation Adjustments [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Term of related contract	[1]
Regulatory liability, current	4,292	[1]	4,292	[1]
Regulatory liability, noncurrent	11,159	[1]	15,450	[1]
Gain From Asset Sales [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	4,903		4,899
Regulatory liability, noncurrent	8,272		13,229
Conservation Programs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year	[2]
Regulatory liability, current	1,875	[2]	2,222	[2]
Regulatory liability, noncurrent	0	[2]	0	[2]
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	2,947		2,396
Regulatory liability, noncurrent	$ 5,809		$ 2,254

[1] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements.

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