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Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Commercial Paper (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Short-term Debt [Line Items] |  |  |  |
Amount outstanding at period end | $ 1,038 | $ 1,038 | $ 814 |
Commercial Paper |  |  |  |
Short-term Debt [Line Items] |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity | 3,250 | 3,250 | 3,250 | $ 2,750
Amount outstanding at period end | 1,038 | 1,038 | 814 | 392
Average amount outstanding | 500 | 788 | 644 | 485
Maximum amount outstanding | $ 1,038 | $ 1,349 | $ 1,247 | $ 1,183
Weighted average interest rate, computed on a daily basis (percentage) | 2.76% | 2.34% | 1.35% | 0.74%
Weighted average interest rate at period end (percentage) | 2.97% | 2.97% | 1.90% | 0.95%
364-Day Term Loan | Xcel Energy Inc. |  |  |  |
Short-term Debt [Line Items] |  |  |  |
Line of Credit Facility, Expiration Period |  | 364 days |  |
Amount outstanding at period end | $ 250 | $ 250 |  |
364-Day Term Loan | Xcel Energy Inc. |  |  |  |
Short-term Debt [Line Items] |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity | $ 500 | $ 500 |  |

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