Commitments and Contingencies, Rate Matters (Details) - NSP-Minnesota - FERC Proceeding, MISO ROE Complaint	1 Months Ended		12 Months Ended
	Feb. 28, 2015	Nov. 30, 2013	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Rate Matters [Abstract]
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%
Public Utilities, Number of steps required			2
Public Utilities, ROE developed with new approach			10.28%
Federal Energy Regulatory Commission (FERC)
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved					10.32%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved				10.82%