Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
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
Designated as Hedging Instrument | 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		0	(2,000)		0		2,000
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		407,000	428,000		1,208,000		1,278,000
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
Designated as Hedging Instrument | 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	[1]	407,000	407,000		1,208,000		1,211,000
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
Designated as Hedging Instrument | 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			(2,000)				2,000
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	[2]		21,000				67,000
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
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		(1,635,000)	(4,848,000)		(8,643,000)		(1,172,000)
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		282,000		7,736,000
Pre-tax gains (losses) recognized during the period in income		(211,000)	(34,000)		(3,013,000)		(3,042,000)
Other Derivative Instruments | Commodity Trading
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	[3]	(211,000)	(28,000)		(23,000)		200,000
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		(1,635,000)	(4,848,000)		(8,643,000)		(1,172,000)
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		282,000	[4]	7,736,000	[4]
Pre-tax gains (losses) recognized during the period in income		0	$ (6,000)	[4]	(2,990,000)	[4]	$ (3,242,000)	[4]
Other Derivative Instruments | Natural Gas Commodity for Electric Generation
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 regulatory (assets) and liabilities		$ 0			$ 900,000

[1] Recorded to interest charges.

[2] Recorded to operating and maintenance (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 for the three and nine months ended Sept. 30, 2017 included no settlement gains or losses and $0.9 million of settlement gains, respectively. Amounts for the three and nine months ended Sept. 30, 2016 included no settlement gains or losses. The remaining derivative settlement gains and losses for the nine months ended Sept. 30, 2017 and 2016 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.