Commitments and Contingencies, Legal Contingencies (Details)	1 Months Ended					12 Months Ended
	Nov. 30, 2017	Sep. 30, 2016	Dec. 31, 2015	Feb. 28, 2015	Nov. 30, 2013	Dec. 31, 2018	Dec. 31, 2017
Gas Trading Litigation
Loss Contingencies [Line Items]
Loss Contingency, Pending Claims, Number						2
Loss Contingency, Settled Claims, number						4
Summary Judgment Granted Against Plaintiff [Member] | Gas Trading Litigation
Loss Contingencies [Line Items]
Loss Contingency, Number of Plaintiffs	2
Remaining in the Litigation [Member] | Gas Trading Litigation
Loss Contingencies [Line Items]
Loss Contingency, Number of Plaintiffs	3
FERC Proceeding, MISO ROE Complaint [Member] | NSP-Minnesota
Loss Contingencies [Line Items]
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) [Member] | FERC Proceeding, MISO ROE Complaint [Member] | NSP-Minnesota
Loss Contingencies [Line Items]
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%