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Fair Value of Financial Instruments (Tables) | 3 Months Ended
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Mar. 31, 2018
Fair Value of Financial Instruments [Abstract] |
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] | The Company’s financial assets and liabilities whose values were recognized at fair value are as follows by level within the fair value hierarchy (in millions):
| As of March 31, 2018
| Level 1 |  | Level 2 |  | Level 3 |  | Other(2) |  | Total
Assets: |  |  |  |  |  |  |  |  |
Cash equivalents | $ | 60 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 60 |
Nuclear decommissioning trust: (1) |  |  |  |  |  |  |  |  |
Debt securities: |  |  |  |  |  |  |  |  |
Domestic government | 3 |  |  | 7 |  |  | — |  |  | — |  |  | 10 |
Corporate credit | — |  |  | 6 |  |  | — |  |  | — |  |  | 6 |
Money market funds measured at NAV (2) | — |  |  | — |  |  | — |  |  | 26 |  |  | 26 |
Non-qualified benefit plan trust: (3) |  |  |  |  |  |  |  |  |
Money market funds | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Equity securities—domestic | 6 |  |  | — |  |  | — |  |  | — |  |  | 6 |
Debt securities—domestic government | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Assets from price risk management activities: (1) (4) |  |  |  |  |  |  |  |  |
Electricity | — |  |  | 2 |  |  | — |  |  | — |  |  | 2 |
Natural gas | — |  |  | 2 |  |  | — |  |  | — |  |  | 2 |
| $ | 71 |  |  | $ | 17 |  |  | $ | — |  |  | $ | 26 |  |  | $ | 114 |
Liabilities from price risk management activities: (1) (4) |  |  |  |  |  |  |  |  |
Electricity | $ | — |  |  | $ | 11 |  |  | $ | 123 |  |  | $ | — |  |  | $ | 134 |
Natural gas | — |  |  | 66 |  |  | 11 |  |  | — |  |  | 77 |
| $ | — |  |  | $ | 77 |  |  | $ | 134 |  |  | $ | — |  |  | $ | 211 |
(1) | Activities are subject to regulation, with certain gains and losses deferred pursuant to regulatory accounting and included in Regulatory assets or Regulatory liabilities as appropriate.
(2) | Assets are measured at NAV as a practical expedient and not subject to hierarchy level classification disclosure.
(3) | Excludes insurance policies of $28 million, which are recorded at cash surrender value.
(4) | For further information, see Note 5, Price Risk Management.
| As of December 31, 2017
| Level 1 |  | Level 2 |  | Level 3 |  | Other (2) |  | Total
Assets: |  |  |  |  |  |  |  |  |
Cash equivalents | $ | 30 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 30 |
Nuclear decommissioning trust: (1) |  |  |  |  |  |  |  |  |
Debt securities: |  |  |  |  |  |  |  |  |
Domestic government | 4 |  |  | 7 |  |  | — |  |  | — |  |  | 11 |
Corporate credit | — |  |  | 6 |  |  | — |  |  | — |  |  | 6 |
Money market funds measured at NAV (2) | — |  |  | — |  |  | — |  |  | 25 |  |  | 25 |
Non-qualified benefit plan trust: (3) |  |  |  |  |  |  |  |  |
Money market funds | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Equity securities—domestic | 7 |  |  | — |  |  | — |  |  | — |  |  | 7 |
Debt securities—domestic government | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Assets from price risk management activities: (1) (4) |  |  |  |  |  |  |  |  |
Electricity | — |  |  | 3 |  |  | — |  |  | — |  |  | 3 |
Natural gas | — |  |  | 3 |  |  | — |  |  | — |  |  | 3 |
| $ | 43 |  |  | $ | 19 |  |  | $ | — |  |  | $ | 25 |  |  | $ | 87 |
Liabilities from price risk management activities: (1) (4) |  |  |  |  |  |  |  |  |
Electricity | $ | — |  |  | $ | 5 |  |  | $ | 130 |  |  | $ | — |  |  | $ | 135 |
Natural gas | — |  |  | 66 |  |  | 9 |  |  | — |  |  | 75 |
| $ | — |  |  | $ | 71 |  |  | $ | 139 |  |  | $ | — |  |  | $ | 210 |
(1) | Activities are subject to regulation, with certain gains and losses deferred pursuant to regulatory accounting and included in Regulatory assets or Regulatory liabilities as appropriate.
(2) | Assets are measured at NAV as a practical expedient and not subject to hierarchy level classification disclosure.
(3) | Excludes insurance policies of $28 million, which are recorded at cash surrender value.
(4) | For further information, see Note 5, Price Risk Management.
Fair Value, Option, Quantitative Disclosures [Table Text Block] | Quantitative information regarding the significant, unobservable inputs used in the measurement of Level 3 assets and liabilities from price risk management activities is presented below:
|  | Fair Value |  | Valuation Technique |  | Significant Unobservable Input |  | Price per Unit
Commodity Contracts |  | Assets |  | Liabilities |  |  |  | Low |  | High |  | Weighted Average
|  | (in millions) |  |  |  |  |  |  |  |  |  |
As of March 31, 2018: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electricity physical forwards |  | $ | — |  |  | $ | 122 |  |  | Discounted cash flow |  | Electricity forward price (per MWh) |  | $ | 6.56 |  |  | $ | 44.31 |  |  | $ | 31.44 |
Natural gas financial swaps |  | — |  |  | 11 |  |  | Discounted cash flow |  | Natural gas forward price (per Decatherm) |  | 1.12 |  |  | 2.58 |  |  | 1.59 |
Electricity financial futures |  | — |  |  | 1 |  |  | Discounted cash flow |  | Electricity forward price (per MWh) |  | 6.56 |  |  | 27.74 |  |  | 18.46 |
|  | $ | — |  |  | $ | 134 |  |  |  |  |  |  |  |  |  |  |
As of December 31, 2017: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electricity physical forwards |  | $ | — |  |  | $ | 130 |  |  | Discounted cash flow |  | Electricity forward price (per MWh) |  | $ | 7.79 |  |  | $ | 41.23 |  |  | $ | 30.95 |
Natural gas financial swaps |  | — |  |  | 9 |  |  | Discounted cash flow |  | Natural gas forward price (per Decatherm) |  | 1.26 |  |  | 2.92 |  |  | 1.90 |
Electricity financial futures |  | — |  |  | — |  |  | Discounted cash flow |  | Electricity forward price (per MWh) |  | 7.79 |  |  | 29.74 |  |  | 21.74 |
|  | $ | — |  |  | $ | 139 |  |  |  |  |  |  |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] | Changes in the fair value of net liabilities from price risk management activities (net of assets from price risk management activities) classified as Level 3 in the fair value hierarchy were as follows (in millions):
| Three Months Ended March 31,
| 2018 |  | 2017
Balance as of the beginning of the period | $ | 139 |  |  | $ | 119 |
Net realized and unrealized (gains)/losses* | (4 | ) |  | 26 |
Transfers out of Level 3 to Level 2 | (1 | ) |  | (1 | )
Balance as of the end of the period | $ | 134 |  |  | $ | 144 |
* Both realized and unrealized (gains)/losses, of which the unrealized portion is fully offset by the effects of regulatory accounting until settlement of the underlying transactions, are recorded in Purchased power and fuel expense in the condensed consolidated statements of income and comprehensive income.

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- Definition Fair Value of Financial Instruments [Abstract] + References No definition available. + Details
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- Definition Tabular disclosure of the fair value measurement of assets using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets) and gains or losses recognized in other comprehensive income (loss), and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs), by class of asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 + Details
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- Definition Tabular disclosure of the application of the fair value option for measurement and reporting of eligible financial assets and liabilities (as defined), as well as certain other eligible items (as defined) included in the statement of financial position, whether such option is elected for a single eligible item or a group of similar eligible items and is in addition to other disclosures concerning fair value which the company may be required to provide. Such disclosure might be expected to include: (1) for items included in the statement of financial position: (a) for each line item in the statement of financial position that includes an item or items for which the fair value option has been elected, provide the aggregate carrying amount of ineligible items included in each line item in the balance sheet, if any; (b) the difference between the aggregate fair value and the aggregate unpaid principal balance (assuming contractual principal amounts and fair value option elected) of: (i) loans and long-term receivables (other than securities otherwise reported at fair value) and (ii) long-term debt instruments; (c) for loans held as assets for which the fair value option has been elected: (i) the aggregate fair value of loans that are 90 days or more past due, (ii) if the policy is to recognize interest income separately from other changes in fair value, the aggregate fair value of loans in nonaccrual status, and (iii) the difference between the aggregate fair value and the aggregate unpaid principal balance for loans that are 90 days or more past due, in nonaccrual status, or both; (2) for items included in the income statement: (a) the amounts of gains and losses from fair value changes included in earnings and in which line in the income statement those gains and losses are reported whether or not combined with gains and losses from items required to be accounted for at fair value; (b) for loans and other receivables held as assets, the estimated amount of gains or losses included in earnings attributable to changes in instrument-specific credit risk; (c) for liabilities with fair values that have been significantly affected during the reporting period by changes in the instrument-specific credit risk, the estimated amount of gains and losses from fair value changes included in earnings that are attributable to changes in the instrument-specific credit risk; and (3) certain other disclosures as required or determined to be provided. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 32 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14217-108612 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (c)(2),(d),(e) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a),(c)(1),(d)(1) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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