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Commitments and Contingencies MISO ROE Complaints (Details) - NSP Minnesota and NSP Wisconsin - FERC Proceeding, MISO ROE Complaint [Member] - Federal Energy Regulatory Commission (FERC) [Member] | 1 Months Ended
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Feb. 28, 2015 | Nov. 30, 2013
Public Utilities, General Disclosures [Line Items] |  |
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates | 12.38% | 12.38%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties | 8.67% | 9.15%

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