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Commitments and Contingencies, Legal Contingencies (Details) | 1 Months Ended | 12 Months Ended
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Jan. 31, 2019 USD ($) | Nov. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2015 | Feb. 28, 2015 | Nov. 30, 2013 | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Gas Trading Litigation |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Loss Contingency, Pending Claims, Number |  |  |  |  |  |  | 2 |  |
Loss Contingency, Settled Claims, number |  |  |  |  |  |  | 4 |  |
NSP-Minnesota |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Percentage of Fault |  |  |  |  |  |  | 48.00% |  |
NSP-Minnesota | Insurance-related Assessments [Member] |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Nuclear Insurance Coverage Exposure Funded By Secondary Financial Protection Program |  |  |  |  |  |  | $ 13,600,000,000 |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Maximum Assessments Per Reactor Per Accident |  |  |  |  |  |  | $ 138,000,000 |  |
PSCo | Line Extension Disputes |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Accrual For Legal Contingency |  |  |  |  |  |  |  | $ 0 |
General Electric (GE) [Domain] |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Percentage of Fault |  |  |  |  |  |  | 52.00% |  |
Minimum | PSCo | Line Extension Disputes |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Loss Contingency, Number of Plaintiffs |  |  |  | 50 |  |  |  |  |
Summary Judgment Granted Against Plaintiff [Member] | Gas Trading Litigation |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Loss Contingency, Number of Plaintiffs |  | 2 |  |  |  |  |  |  |
Remaining in Litigation [Member] | Gas Trading Litigation |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Loss Contingency, Number of Plaintiffs |  | 3 |  |  |  |  |  |  |
FERC Proceeding, MISO ROE Complaint [Member] | NSP-Minnesota |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Public Utilities, Number of steps required |  |  |  |  |  |  | 2 |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates |  |  |  |  | 12.38% | 12.38% |  |  |
Public Utilities, ROE developed with new approach |  |  |  |  |  |  | 10.28% |  |
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties |  |  |  |  | 8.67% | 9.15% |  |  |
Federal Energy Regulatory Commission (FERC) [Member] | FERC Proceeding, MISO ROE Complaint [Member] | NSP-Minnesota |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved |  |  | 10.32% | 10.32% |  |  |  |  |
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved |  |  |  |  |  |  |  | 10.82% |
Southwest Power Pool (SPP) [Member] | SPP Open Access Transmission Tariff Upgrade Costs [Member] | SPS |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Public Utilities, Billed Charges For Transmission Service Upgrades |  |  |  |  |  |  |  |  | $ 13,000,000
Subsequent Event | NSP-Minnesota |  |  |  |  |  |  |  |  |
Legal Contingencies [Abstract] |  |  |  |  |  |  |  |  |
Customer refund of previously recovered purchased power costs | $ 20,000,000 |  |  |  |  |  |  |  |

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