Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges	$ 0		$ 0		$ 0		$ 0
Recognized gains (losses) from fair value hedges or related hedged transactions	0		0		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	(73,000)		(58,891,000)		(48,000)		(17,008,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	1,145,000		340,000		2,270,000		677,000
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0		0		0
Pre-tax gains (losses) recognized during the period in income	0		0		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		(58,695,000)		0		(16,991,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	1,162,000	[1]	389,000	[1]	2,312,000	[1]	777,000	[1]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0		0		0
Pre-tax gains (losses) recognized during the period in income	0		0		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	(73,000)		(196,000)		(48,000)		(17,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	(17,000)	[2]	(49,000)	[2]	(42,000)	[2]	(100,000)	[2]
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0		0		0
Pre-tax gains (losses) recognized during the period in income	0		0		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		0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	50,547,000		39,059,000		57,019,000		29,858,000
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	(13,764,000)		(9,713,000)		(28,984,000)		63,254,000
Pre-tax gains (losses) recognized during the period in income	(742,000)		1,589,000		2,050,000		3,203,000
Other Derivative Instruments [Member] | Commodity Trading [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		0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0		0		0
Pre-tax gains (losses) recognized during the period in income	(498,000)	[3]	1,589,000	[3]	2,278,000	[3]	3,312,000	[3]
Other Derivative Instruments [Member] | Electric 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	0		0		0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	53,974,000		38,174,000		60,393,000		39,756,000
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	(13,764,000)	[4]	(9,713,000)	[4]	(28,993,000)	[4]	(17,685,000)	[4]
Pre-tax gains (losses) recognized during the period in income	0		0		0		0
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 fair value gains (losses) recognized during the period in accumulated other comprehensive loss	0		0		0		0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities	(3,427,000)		885,000		(3,374,000)		(9,898,000)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss	0		0		0		0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	0		0		9,000	[5]	80,939,000	[5]
Pre-tax gains (losses) recognized during the period in income	(244,000)	[4]	0		(228,000)	[4]	(109,000)	[4]
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 (gains) losses reclassified into income during the period from regulatory assets and (liabilities)							$ 5,000,000

[1] Amounts are recorded to interest charges.

[2] Amounts are recorded to O&M expenses.

[3] Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.

[4] Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.

[5] Amounts for the six months ended June 30, 2012 included $5.0 million of settlement losses on derivatives entered 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 six months ended June 30, 2013 were immaterial. The remaining settlement losses for the six months ended June 30, 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 out of income to a regulatory asset, as appropriate.