Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	$ 239		$ 221
Regulatory Liability, Noncurrent		5,083		1,383
Excess deferred taxes - TCJA
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0	[2]	0
Regulatory Liability, Noncurrent		3,733	[2]	0
Revaluation of Non-plant ADIT	[2]	$ 174
Regulatory liability, remaining amortization period		Various
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0		0
Regulatory Liability, Noncurrent		$ 1,131		1,135
Regulatory liability, remaining amortization period		Plant lives
Renewable Resources and Environmental Initiatives
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 19		5
Regulatory Liability, Noncurrent		$ 56		71
Regulatory liability, remaining amortization period		Various
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0		0
Regulatory Liability, Noncurrent		$ 42		45
Regulatory liability, remaining amortization period		Various
Deferred Income Tax Adjustment
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 0		0
Regulatory Liability, Noncurrent		$ 38		48
Regulatory liability, remaining amortization period		Various
Deferred Electric, Natural Gas and Steam Production Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 104		98
Regulatory Liability, Noncurrent		$ 0		0
Regulatory liability remaining amortization period, maximum		1 year
Contract Valuation Adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[3]	$ 30		22
Regulatory Liability, Noncurrent	[3]	$ 0		2
Regulatory liability, remaining amortization period		Term of related contract
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[4]	$ 23		25
Regulatory Liability, Noncurrent	[4]	$ 0		0
Regulatory liability remaining amortization period, maximum		1 year
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 18		20
Regulatory Liability, Noncurrent		$ 0		0
Regulatory liability remaining amortization period, maximum		1 year
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 45		51
Regulatory Liability, Noncurrent		$ 83		82
Regulatory liability, remaining amortization period		Various
Other Current Liabilities
Regulatory Liabilities [Line Items]
Entity's Recorded Provision for Revenue Subject To Refund		$ 15		$ 46

[1] Revenue subject to refund of $15 million and $46 million for 2017 and 2016, respectively, is included in other current liabilities.

[2] Primarily relates to the revaluation of recoverable/regulated plant ADIT and $174 million revaluation impact of non-plant ADIT at Dec. 31, 2017.

[3] Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.

[4] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.