Commitments and Contingencies Nuclear Obligations Phantom (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Public Utilities, General Disclosures [Line Items]
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%
Approved annual accrual for decommissioning costs	$ 14	$ 14	$ 14
Measurement Input, Risk Free Interest Rate
Public Utilities, General Disclosures [Line Items]
Debt Instrument, Measurement Input	0.0239	0.0333

Value of input used to measure debt instrument, including, but not limited to, convertible and non-convertible debt.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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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.

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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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