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Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Regulatory Assets [Line Items] |  |
Regulatory Assets | $ 462 | $ 500
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 383 |
Regulatory Liabilities | 1,391 | 1,319
Regulatory Assets Earning a Rate of Return at the Approved Rate | 1,384 |
Removal Costs [Member] |  |
Regulatory Assets [Line Items] |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0 |
Regulatory Liabilities | 979 | 933
Deferred Income Tax Charge [Member] |  |
Regulatory Assets [Line Items] |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0 |
Regulatory Liabilities | $ 267 | 277
Environmental Restoration Costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Current Asset, End Date for Recovery | 2019 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 0 |
Regulatory Liabilities | 1 | 3
Asset Retirement Obligation Costs [Member] |  |
Regulatory Assets [Line Items] |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0 |
Regulatory Liabilities | $ 53 | 52
Revenue Subject to Refund [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Current Asset, End Date for Recovery | 2020 |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 0 |
Regulatory Liabilities | 45 | 0
Other Regulatory Assets (Liabilities) [Member] |  |
Regulatory Assets [Line Items] |  |
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 7 |
Regulatory Liabilities | 46 | 54
Regulatory Assets Earning a Rate of Return at the Approved Rate | 39 |
Other Regulatory Assets Earning a Return [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | 53 |
Earning a return [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | $ 79 |
Deferred Derivative Gain (Loss) [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Current Asset, End Date for Recovery | 2035 |
Regulatory Assets | $ 0 | 204
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 131 |
Pension and Other Postretirement Plans Costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | 0 | 218
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 222 |
Loss on Reacquired Debt [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Current Asset, End Date for Recovery | 2036 |
Regulatory Assets | $ 0 | 19
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 16 |
Environmental Restoration Costs [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Current Asset, End Date for Recovery | 2039 |
Regulatory Assets | $ 26 | 0
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | 0 |
Other Regulatory Assets (Liabilities) [Member] |  |
Regulatory Assets [Line Items] |  |
Regulatory Assets | 67 | $ 59
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided | $ 14 |

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- Definition 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. + References No definition available. + Details
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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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- Definition The end date for recovery of the individual regulatory current asset in a schedule of regulatory current assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
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- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82849221&loc=d3e44360-110383 + Details
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