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Summary of Significant Accounting Policies (Narrative II) (Details) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2024 CAD ($) | Sep. 30, 2024 USD ($) | Dec. 31, 2024 CAD ($) Item | Dec. 31, 2024 USD ($) Item | Dec. 31, 2023 CAD ($) | Aug. 05, 2024 CAD ($) | Dec. 31, 2022 CAD ($)
Asset Impairment Charges |  |  |  |  |  |  |
Impairment charge |  |  | $ 225,000,000 |  | $ 0 |  |
Goodwill |  |  |  |  |  |  |
Goodwill |  |  | 5,858,000,000 |  | 5,871,000,000 |  | $ 6,012,000,000
Goodwill impairment charge |  |  | $ 214,000,000 |  | 0 |  |
Lease, Practical Expedient, Lessor Single Lease Component [true false] |  |  | true |  |  |  |
New Mexico Gas Company [Member] |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | $ 366,000,000 |
Goodwill impairment charge | $ 210,000,000 | $ 155 | $ 210,000,000 | $ 155 |  |  |
Goodwill impairment charge, net of tax | 198,000,000 | $ 146 | 198,000,000 | $ 146 |  |  |
Various Canadian And US Government Agencies [Member] |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |
Amount received from government funds |  |  | $ 47,000,000 |  | $ 7,000,000 |  |
Government Assistance Asset Increase Noncurrent Statement Of Financial Position Extensible Enumeration |  |  | Property, Plant and Equipment, Net | Property, Plant and Equipment, Net | Property, Plant and Equipment, Net |  |
Natural Resources Canada Smart Renewables & Electrification Pathways [Member] |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |
Number of battery storage systems funded for construction | Item |  |  | 3 | 3 |  |  |
Percent under grant for claims of eligible project costs |  |  | 33.00% | 33.00% |  |  |
Maximum project costs eligible under grant for claims |  |  | $ 109,000,000 |  |  |  |
Amount received from government funds |  |  | $ 26,000,000 |  |  |  |
Government Assistance Asset Increase Noncurrent Statement Of Financial Position Extensible Enumeration |  |  | Property, Plant and Equipment, Net | Property, Plant and Equipment, Net | Property, Plant and Equipment, Net |  |
Long-Lived Assets |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |
Impairment charge |  |  | $ 11,000,000 |  | $ 0 |  |
Certain Long Lived Assets |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |
Impairment charge |  |  | 19,000,000 |  |  |  |
Asset Impairment Charges, net of tax |  |  | 14,000,000 |  |  |  |
Included In Other Income, Net |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |
Impairment charge |  |  | 8,000,000 |  |  |  |
Equity Method Investments |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |
Impairment charge |  |  | 0 |  | 0 |  |
Financial Assets |  |  |  |  |  |  |
Asset Impairment Charges |  |  |  |  |  |  |
Impairment charge |  |  | 0 |  | $ 0 |  |
NMGC |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |
Goodwill | 303,000,000 |  | $ 303,000,000 |  |  |  |
Goodwill impairment charge | $ 210,000,000 |  |  |  |  |  |

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