Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
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
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	8,000		(31,000)		3,000		(20,000)
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	(189,000)		(189,000)		(382,000)		(381,000)
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 | Interest Rate
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	(182,000)	[1]	(182,000)	[1]	(362,000)	[1]	(362,000)	[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 | Vehicle Fuel And Other Commodity
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	8,000		(31,000)		3,000		(20,000)
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	(7,000)	[2]	(7,000)	[2]	(20,000)	[2]	(19,000)	[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
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	(1,916,000)		(3,211,000)		7,910,000		(3,169,000)
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		(8,579,000)		7,000
Pre-tax gains (losses) recognized during the period in income	(65,000)		(244,000)		(4,381,000)		(228,000)
Other Derivative Instruments | Natural Gas Commodity
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	(1,916,000)		(3,211,000)		7,910,000		(3,169,000)
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		(8,579,000)	[3]	7,000	[3]
Pre-tax gains (losses) recognized during the period in income	$ (65,000)	[4]	$ (244,000)	[4]	$ (4,381,000)	[3]	$ (228,000)	[4]

[1] Recorded to interest charges.

[2] Recorded to O&M expenses.

[3] Amounts for the six months ended June 30, 2014 and 2013 included immaterial 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. The remaining derivative settlement gains and losses for the six months ended June 30, 2014 and 2013 relate to natural gas operations and are recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.

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