Fair Value of Financial Assets and Liabilities, Impact of Derivative Activity (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financial Impact of Qualifying Fair Value Hedges on Earnings [Abstract]
Derivative instruments designated as fair value hedges		$ 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		(13,000,000)		(22,000,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0		0
Pre-tax (gains) losses reclassified from accumulated OCI into income		1,000,000	1,000,000	2,000,000	1,000,000
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		(13,000,000)		(22,000,000)
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		0		0
Pre-tax (gains) losses reclassified from accumulated OCI into income		1,000,000	1,000,000	2,000,000	1,000,000
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
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		24,000,000	37,000,000	2,000,000	8,000,000
Pre-tax (gains) losses reclassified from accumulated OCI into income		0	0	0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)		0	(3,000,000)	0	2,000,000
Pre-tax gains (losses) recognized during the period in income		5,000,000	2,000,000	0	8,000,000
Other Derivative Instruments | Commodity Trading
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified from accumulated OCI into income		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	[1]	5,000,000	2,000,000	4,000,000	10,000,000
Other Derivative Instruments | Electric 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		26,000,000	37,000,000	4,000,000	8,000,000
Pre-tax (gains) losses reclassified from accumulated OCI into income			0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	[2]		(3,000,000)	1,000,000
Pre-tax gains (losses) recognized during the period in income			0	0
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
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(2,000,000)		(2,000,000)
Pre-tax (gains) losses reclassified from accumulated OCI into income				0	0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities)	[3]			(1,000,000)	2,000,000
Pre-tax gains (losses) recognized during the period in income	[3]			(4,000,000)	(2,000,000)
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	$ 0	$ 0	$ (1,000,000)

[1] 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.

[2] 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.

[3] Amounts for both the three and six months ended June 30, 2019 included no settlement gains or 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. Amounts for the three and six months ended June 30, 2018 included no such settlement gains or losses and $1 million of such settlement losses, respectively. Remaining settlement losses for the three and six months ended June 30, 2019 and 2018 related to natural gas operations and were 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.