Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0		$ 0
Recognized gains (losses) from fair value hedges or related hedged transactions	0		0
Cash Flow Hedges [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	11,000		20,992,000
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	(192,000)		(605,000)
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0
Pre-tax gains (losses) recognized during the period in income	0		0
Cash Flow Hedges [Member] | Interest Rate [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	0		20,917,000
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	(180,000)	[1]	(582,000)	[1]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0
Pre-tax gains (losses) recognized during the period in income	0		0
Cash Flow Hedges [Member] | Vehicle Fuel And Other Commodity [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	11,000		75,000
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	(12,000)	[2]	(23,000)	[2]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0
Pre-tax gains (losses) recognized during the period in income	0		0
Other Derivative Instruments [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	43,000		(7,715,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	7,000		61,858,000
Pre-tax gains (losses) recognized during the period in income	16,000		(109,000)
Other Derivative Instruments [Member] | Natural Gas Commodity [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)			5,000,000
Other Derivative Instruments [Member] | Natural Gas Commodity for Electric Generation [Member]
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss	0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	43,000		(7,715,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	7,000	[3]	61,858,000	[3]
Pre-tax gains (losses) recognized during the period in income	$ 16,000	[4]	$ (109,000)	[4]

[1] Amounts are recorded to interest charges.

[2] Amounts are recorded to operating and maintenance (O&M) expenses.

[3] Amounts for the three months ended March 31, 2012 included $5.0 million of settlement losses on derivatives utilized to mitigate natural gas price risk for electric generation, recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. Such losses for the three months ended March 31, 2013 were immaterial. The remaining settlement losses for the three months ended March 31, 2013 and 2012 relate to natural gas operations and are recorded to cost of natural gas sold and transported. These losses are subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate.

[4] Amounts are recorded to electric fuel and purchased power.