Commitments and Contingencies, Removal Costs (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Decommissioning Fund Investments, Fair Value	$ 2,055
Percentage Of Total Obligation For Decommissioning Expected To Be Funded By External Funds	100.00%
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 1,175	$ 1,131
Plant Removal Costs | NSP-Minnesota
Regulatory Liabilities [Line Items]
Regulatory liabilities	485	442
Plant Removal Costs | PSCo
Regulatory Liabilities [Line Items]
Regulatory liabilities	344	346
Plant Removal Costs | SPS
Regulatory Liabilities [Line Items]
Regulatory liabilities	188	197
Plant Removal Costs | NSP-Wisconsin
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 158	$ 146