Commitments and Contingencies Commitments and Contingencies, Rate Matters, NSP-Minnesota (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	Jan. 31, 2019	Feb. 28, 2015	Nov. 30, 2013	Dec. 31, 2018	Dec. 31, 2016
General Electric (GE) [Domain]
Rate Matters [Abstract]
Percentage of Fault				52.00%
NSP-Minnesota
Rate Matters [Abstract]
Percentage of Fault				48.00%
Customer refund of previously recovered purchased power costs	$ 20
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint
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, ROE developed with new approach				10.28%
Public Utilities, Number of steps required				2
NSP-Minnesota | FERC Proceeding, MISO ROE Complaint | Federal Energy Regulatory Commission (FERC)
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved					10.82%
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved					10.32%