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Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 464 | $ 424
Regulatory Asset, Noncurrent |  | 3,326 | 3,005
Past expenditures not currently earning a return |  | 178 | 212
Pension and Retiree Medical Obligations |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | 87 | 91
Regulatory Asset, Noncurrent |  | $ 1,500 | 1,499
Regulatory asset, remaining amortization period |  | Various |
Net AROs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current | [1] | $ 0 | 0
Regulatory Asset, Noncurrent | [1] | $ 452 | 301
Regulatory asset, remaining amortization period |  | Plant lives |
Excess deferred taxes - TCJA |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 0 | 0
Regulatory Asset, Noncurrent |  | $ 296 | 254
Regulatory asset, remaining amortization period |  | Various |
Recoverable Deferred Taxes on AFUDC Recorded in Plant |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 0 | 0
Regulatory Asset, Noncurrent |  | $ 264 | 244
Regulatory asset, remaining amortization period |  | Plant lives |
Environmental Remediation Costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 17 | 16
Regulatory Asset, Noncurrent |  | $ 155 | 165
Regulatory asset, remaining amortization period |  | Various |
Depreciation Differences |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 18 | 20
Regulatory Asset, Noncurrent |  | $ 107 | 69
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 13 years |
Benson purchase power agreement termination and asset purchase |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 10 | 0
Regulatory Asset, Noncurrent |  | $ 86 | 0
Regulatory asset, remaining amortization period |  | 10 years |
Contract Valuation Adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current | [2] | $ 17 | 21
Regulatory Asset, Noncurrent | [2] | $ 77 | 93
Regulatory asset, remaining amortization period |  | Term of related contract |
Laurentian Energy Authority purchase power agreement termination |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 18 | 0
Regulatory Asset, Noncurrent |  | $ 73 | 0
Regulatory asset, remaining amortization period |  | 5 years |
Purchased Power Agreements |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 4 | 3
Regulatory Asset, Noncurrent |  | $ 63 | 67
Regulatory asset, remaining amortization period |  | Term of related contract |
Prairie Island Nuclear Plant Extended Power Uprate |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 3 | 3
Regulatory Asset, Noncurrent |  | $ 56 | 58
Regulatory asset, remaining amortization period |  | 16 years |
Losses on Reacquired Debt |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 4 | 5
Regulatory Asset, Noncurrent |  | $ 44 | 48
Regulatory asset, remaining amortization period |  | Term of related contract |
State Commission Adjustments |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 1 | 1
Regulatory Asset, Noncurrent |  | $ 29 | 29
Regulatory asset, remaining amortization period |  | Plant lives |
Conservation Programs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current | [3] | $ 42 | 50
Regulatory Asset, Noncurrent | [3] | $ 28 | 32
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 2 years |
Property Tax |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 14 | 8
Regulatory Asset, Noncurrent |  | $ 10 | 24
Regulatory asset, remaining amortization period |  | Various |
Nuclear Refueling Outage Costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 37 | 49
Regulatory Asset, Noncurrent |  | $ 14 | 20
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 2 years |
Deferred Purchased Natural Gas and Electric Energy Costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 57 | 21
Regulatory Asset, Noncurrent |  | $ 13 | 13
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 3 years |
Renewable Resources and Environmental Initiatives |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 39 | 48
Regulatory Asset, Noncurrent |  | $ 9 | 10
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 2 years |
Sales True Up and Revenue Decoupling |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 38 | 37
Regulatory Asset, Noncurrent |  | $ 7 | 12
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 2 years |
Gas Pipeline Inspection and Remediation Costs |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 28 | 24
Regulatory Asset, Noncurrent |  | $ 3 | 12
Regulatory Asset Amortization Period, minimum |  | 1 year |
Regulatory Asset Amortization Period, maximum |  | 2 years |
Other Regulatory Assets |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Asset, Current |  | $ 30 | 27
Regulatory Asset, Noncurrent |  | $ 40 | $ 55
Regulatory asset, remaining amortization period |  | Various |
[1] | Includes amounts recorded for future recovery of AROs, less amounts recovered through nuclear decommissioning accruals and gains from decommissioning investments.
[2] | Includes the fair value of certain long-term PPAs used to meet energy capacity requirements and valuation adjustments on natural gas commodity purchases.
[3] | Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.

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- Definition Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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- Definition Remaining balance of regulatory assets for which no return on investment during recovery is provided. In some cases, a regulator may permit an enterprise to include a cost that would be charged to expense by an unregulated enterprise as an allowable cost over a period of time by amortizing that cost for rate-making purposes, but the regulator does not include the unrecovered amount in the rate base. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 55 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82849221&loc=d3e44360-110383 + Details
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- Definition The maximum amortization period for recovery of the individual regulatory asset in a schedule of regulatory assets. + References No definition available. + Details
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- Definition The minimum amortization period for recovery of the individual regulatory asset in a schedule of regulatory assets. + References No definition available. + Details
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