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Commitments and Contingencies NSP-Minnesota - MEC Transactions (Details) $ in Millions		6 Months Ended	9 Months Ended
	Jan. 13, 2020 USD ($)	Jun. 30, 2020 MW	Sep. 30, 2020 USD ($)	Jul. 10, 2020 USD ($)	Dec. 31, 2019 USD ($)
Public Utilities, General Disclosures [Line Items]
Other Liabilities, Current			$ 416		$ 468
Liabilities, Current			4,393		$ 4,568
Gain on Sale				$ 20
MEC Holdings LLC \| Unregulated Operation [Member]
Public Utilities, General Disclosures [Line Items]
Facility Output \| MW		760
PP&E Additions	$ 650
MEC Holdings LLC \| Unregulated Operation [Member] \| Southwest Generation [Domain]
Public Utilities, General Disclosures [Line Items]
PP&E Sale			$ 684