Commitments and Contingencies Commitments and Contingencies, Rate Matters, NSP-Minnesota (Details) - NSP-Minnesota - FERC Proceeding, MISO ROE Complaint	1 Months Ended				12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Feb. 28, 2015	Nov. 30, 2013	Dec. 31, 2018	Dec. 31, 2017
Rate Matters [Abstract]
Public Utilities, Base Return On Equity Charged To Customers Through Transmission Formula Rates			12.38%	12.38%
Public Utilities, ROE developed with new approach					10.28%
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
Federal Energy Regulatory Commission (FERC)
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved	10.32%	10.32%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved						10.82%