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Nuclear Obligations (Details) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) Canister | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Public Utilities, General Disclosures [Line Items] |  |  |
Decommissioning Fund Investments, Fair Value | $ 2,055 |  |
Fuel Disposal [Abstract] |  |  |
DOE fuel disposal assessments included in fuel expense |  | $ 0 |
Regulatory Plant Decommissioning Recovery [Abstract] |  |  |
Assumed annual escalation rate during plant removal activities | 4.36% | 4.36% |
Assumed annual escalation rate during spent fuel management and site restoration activities | 3.36% | 3.36% |
Percentage Of Total Obligation For Decommissioning Expected To Be Funded By External Funds | 100.00% |  |
Approved annual accrual for decommissioning costs | $ 14 |  |
Funded Status of Nuclear Decommissioning Obligation [Abstract] |  |  |
Estimated decommissioning cost obligation from most recently approved study (in 2014 dollars) | 3,012 | $ 3,012 |
Effect of escalating costs | 539 | 396 |
Estimated decommissioning cost obligation (in current dollars) | 3,551 | 3,408 |
Effect of escalating costs to payment date | 7,654 | 7,797 |
Estimated future decommissioning costs (undiscounted) | 11,205 | 11,205 |
Effect of discounting obligation (using average risk-free interest rate of 3.33% and 2.80% for 2018 and 2017, respectively) | (6,911) | (6,398) |
Discounted decommissioning cost obligation | 4,294 | 4,807 |
Underfunding of external decommissioning fund compared to the discounted decommissioning obligation | (2,239) | (2,664) |
Regulatory Basis to GAAP Basis Reconciliation [Abstract] |  |  |
Differences in Discount Rate and Market Risk Premium | (1,447) | (1,403) |
Operating and Maintenance Costs Not Included for GAAP | (879) | (1,041) |
ARO differences between 2017 and 2014 cost studies | 0 | (489) |
Asset Retirement Obligation | 2,568 | 2,475 | $ 2,782
Annual Decommissioning Recorded As Depreciation Expense [Abstract] |  |  |
Annual decommissioning recorded as depreciation expense: (a) (b) | $ 20 | 20 | 20
Minimum |  |  |
Regulatory Plant Decommissioning Recovery [Abstract] |  |  |
Assumed after tax rate of return used to determine funding for external decommissioning trust fund | 5.23% |  |
Maximum |  |  |
Regulatory Plant Decommissioning Recovery [Abstract] |  |  |
Assumed after tax rate of return used to determine funding for external decommissioning trust fund | 6.30% |  |
Nuclear Plant [Member] |  |  |
Regulatory Basis to GAAP Basis Reconciliation [Abstract] |  |  |
Asset Retirement Obligation | $ 1,968 | 1,874 | $ 2,249
Monticello [Member] |  |  |
Fuel Disposal [Abstract] |  |  |
Number of authorized canisters filled and placed In dry cask nuclear storage facility | Canister | 30 |  |
Prairie Island [Member] |  |  |
Fuel Disposal [Abstract] |  |  |
Number of authorized canisters filled and placed In dry cask nuclear storage facility | Canister | 44 |  |
Number of authorized canisters in dry cask nuclear storage facility | Canister | 64 |  |
Fair Value | Fair Value Measured on a Recurring Basis | Nuclear Decommissioning Fund |  |  |
Funded Status of Nuclear Decommissioning Obligation [Abstract] |  |  |
Assets Held in External Decommissioning Trust |  | $ 2,143 |
Measurement Input, Risk Free Interest Rate |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |
Measurement input (as a percent) | 0.0333 | 0.0280 |

X
- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Annual accrual for decommissioning costs approved by the Minnesota Public Utility Commission for 2013 for Minnesota retail customers. + References No definition available. + Details
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- Definition Assumed rate of annual increase in decommissioning costs during independent fuel storage installation and site restoration portion of decommissioning based on periodic site-specific cost studies. + References No definition available. + Details
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- Definition Assumed rate of annual increase in decommissioning costs during operations and radiological portion of decommissioning based on periodic site-specific cost studies. + References No definition available. + Details
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- Definition The assumed after-tax rate of return used in the annuity approach to determine funding for the external nuclear decommissioning trust fund. + References No definition available. + Details
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- Definition Amount paid by the entity to the Department of Energy during the period in connection with the Department of Energy's permanent nuclear disposal program. + References No definition available. + Details
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- Definition Amount of a regulatory decommissioning obligation associated with a legal obligation that a firm incurs when it constructs or operates an asset that must eventually be withdrawn from active service. This may include operating assets such as nuclear power plants or shipping vessels. + References No definition available. + Details
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- Definition Net amount of discounted decommissioning cost obligation compared to assets held in external decommissioning trust. + References No definition available. + Details
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- Definition Decrease in estimated decommissioning cost obligation due to discounting the obligation using a risk-free interest rate. + References No definition available. + Details
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- Definition Estimated decommissioning cost obligation based on most recently approved periodic site-specific cost study. + References No definition available. + Details
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- Definition -- None. No documentation exists for this element. -- + References No definition available. + Details
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- Definition Amount of nuclear decommissioning expense reported as depreciation expense. + References No definition available. + Details
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- Definition Number of canisters authorized by the Minnesota Public Utility Commission that have been filled and placed in the Monticello dry-cask nuclear storage facility. + References No definition available. + Details
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- Definition Number of canisters authorized by the Minnesota Public Utility Commission to be placed in the Monticello dry-cask nuclear storage facility. + References No definition available. + Details
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- Definition Operating and Maintenance Costs Not Included for GAAP + References No definition available. + Details
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- Definition Percentage of total obligation for decommissioning nuclear facilities expected to be funded by external decommissioning trust fund. + References No definition available. + Details
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- Definition Regulatory Basis to GAAP Basis Reconciliation [Abstract] + References No definition available. + Details
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