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Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Remaining Recovery Period of Regulatory Assets, Price Risk Management	6 years
Regulatory Assets, Current	$ 62	$ 36
Regulatory Assets, Noncurrent	438	498
Regulatory Liability, Current	31	51
Regulatory Liability, Noncurrent	$ 1,288	958
Remaining Recovery Period of Regulatory Asset, Debt Reacquisition Costs	6 years
Remaining refund period of regulatory liability, Trojan decommissioning activities	5 years
Removal Costs [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	$ 0	0
Regulatory Liability, Noncurrent	933	887
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	28	33
Regulatory Liability, Noncurrent	26	22
Asset Retirement Obligation Costs [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	52	49
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	3	18
Regulatory Liability, Noncurrent	0	0
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	277	0
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	9	10
Regulatory Assets, Noncurrent	50	35
Loss on Reacquired Debt [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	19	22
Deferred Derivative Gain (Loss) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	53	26
Regulatory Assets, Noncurrent	151	120
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	218	235
Deferred Income Tax Charge [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	0	0
Regulatory Assets, Noncurrent	$ 0	$ 86

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Remaining Recovery Period of Regulatory Asset, Debt Issuance Costs

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Remaining recovery period of regulatory assets, Price Risk Management, for which no return on investment during recovery period is provided.

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Remaining refund period of regulatory liability, Trojan decommissioning activities

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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