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Fair Value of Financial Instruments (Tables) | 9 Months Ended
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Sep. 30, 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 September 30, 2018
| Level 1 |  | Level 2 |  | Level 3 |  | Other(2) |  | Total
Assets: |  |  |  |  |  |  |  |  |
Cash equivalents | $ | 170 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 170 |
Nuclear decommissioning trust: (1) |  |  |  |  |  |  |  |  |
Debt securities: |  |  |  |  |  |  |  |  |
Domestic government | 8 |  |  | 16 |  |  | — |  |  | — |  |  | 24 |
Corporate credit | — |  |  | 12 |  |  | — |  |  | — |  |  | 12 |
Money market funds measured at NAV (2) | — |  |  | — |  |  | — |  |  | 6 |  |  | 6 |
Non-qualified benefit plan trust: (3) |  |  |  |  |  |  |  |  |
Money market funds | 3 |  |  | — |  |  | — |  |  | — |  |  | 3 |
Equity securities—domestic | 6 |  |  | — |  |  | — |  |  | — |  |  | 6 |
Debt securities—domestic government | 1 |  |  | — |  |  | — |  |  | — |  |  | 1 |
Assets from interest rate swap derivatives | — |  |  | 6 |  |  | — |  |  | — |  |  | 6 |
Assets from price risk management activities: (1) (4) |  |  |  |  |  |  |  |  |
Electricity | — |  |  | 4 |  |  | 1 |  |  | — |  |  | 5 |
Natural gas | — |  |  | 1 |  |  | — |  |  | — |  |  | 1 |
| $ | 188 |  |  | $ | 39 |  |  | $ | 1 |  |  | $ | 6 |  |  | $ | 234 |
Liabilities from price risk management activities: (1) (4) |  |  |  |  |  |  |  |  |
Electricity | $ | — |  |  | $ | 4 |  |  | $ | 112 |  |  | $ | — |  |  | $ | 116 |
Natural gas | — |  |  | 36 |  |  | 14 |  |  | — |  |  | 50 |
| $ | — |  |  | $ | 40 |  |  | $ | 126 |  |  | $ | — |  |  | $ | 166 |
(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 $29 million, which are recorded at cash surrender value.
(4) | For further information, see Note 5, 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, Risk Management.
Fair Value Option, 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 September 30, 2018: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Electricity physical forwards |  | $ | — |  |  | $ | 112 |  |  | Discounted cash flow |  | Electricity forward price (per MWh) |  | $ | 12.34 |  |  | $ | 49.11 |  |  | $ | 33.83 |
Natural gas financial swaps |  | — |  |  | 14 |  |  | Discounted cash flow |  | Natural gas forward price (per Decatherm) |  | 1.02 |  |  | 3.21 |  |  | 1.68 |
Electricity financial futures |  | 1 |  |  | — |  |  | Discounted cash flow |  | Electricity forward price (per MWh) |  | 14.90 |  |  | 30.75 |  |  | 25.15 |
|  | $ | 1 |  |  | $ | 126 |  |  |  |  |  |  |  |  |  |  |
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 September 30, |  | Nine Months Ended September 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Balance as of the beginning of the period | 129 |  |  | 153 |  |  | $ | 139 |  |  | $ | 119 |
Net realized and unrealized (gains)/losses* | (2 | ) |  | (1 | ) |  | (10 | ) |  | 34 |
Transfers out of Level 3 to Level 2 | (2 | ) |  | 1 |  |  | (4 | ) |  | — |
Balance as of the end of the period | $ | 125 |  |  | $ | 153 |  |  | $ | 125 |  |  | $ | 153 |
* 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 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19279-110258 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of information about asset and liability measured at fair value under fair value option. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 31 -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14210-108612 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14217-108612 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19190-110258 + Details
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