Rate Matters, NSP-Minnesota (Details) - NSP-Minnesota $ in Millions	1 Months Ended								2 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	Nov. 30, 2017 USD ($)	Sep. 30, 2016	Jun. 30, 2016	Feb. 28, 2015	Nov. 30, 2013	Mar. 31, 2018 USD ($)	Mar. 31, 2018	Dec. 31, 2017 USD ($) MW
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%
Minnesota Public Utilities Commission | Gas Utility Infrastructure Cost Rider 2017 [Member]
Rate Matters [Abstract]
Public Utilities, Approved Rider Revenue, Amount			$ 20
Minnesota Public Utilities Commission | Gas Utility Infrastructure Cost Rider 2018 [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount				$ 28
Public Utilities, Revised Requested Rider Revenue		$ 24
Minnesota Public Utilities Commission | MPUC Proceeding - 2017 and 2018 Renewable Energy Standard Rider - 2017 Rates [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount											$ (10)
Public Utilities, Revised Requested Rider Revenue									$ (13)
Minnesota Public Utilities Commission | MPUC Proceeding - 2017 and 2018 Renewable Energy Standard Rider [Member]
Rate Matters [Abstract]
Public Utilities, Requested Return on Equity, Percentage											10.00%
Public Utilities, Facility Generating Capacity | MW											1,550
Minnesota Public Utilities Commission | MPUC Proceeding - 2017 and 2018 Renewable Energy Standard Rider - 2018 Rates [Member]
Rate Matters [Abstract]
Public Utilities, Requested Rider Revenue, Amount											$ 11
Public Utilities, Revised Requested Rider Revenue									$ 23
Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Approved					10.32%
Public Utilities, Length of Refund Period, In Months					15 months
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved										10.82%
Public Utilities, ROE Basis Point Adder, Approved										50
Administrative Law Judge | FERC Proceeding, MISO ROE Complaint
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties						9.70%
Subsequent Event | Minnesota Public Utilities Commission | MPUC Proceeding - Tax Cuts and Jobs Act of 2017 [Member]
Rate Matters [Abstract]
Tax Cuts and Jobs Act of 2017, Estimated 2018 Electric Rate Reduction	$ 136
Tax Cuts and Jobs Act of 2017, Estimated 2018 Natural Gas Rate Reduction	7
Tax Cuts and Jobs Act of 2017, Proposed 2018 Electric Refund of TCJA Benefits	68
Tax Cuts and Jobs Act of 2017, Proposed Electric Deferral of TCJA Benefits	44
Tax Cuts and Jobs Act of 2017, Proposed Amount of Accelerated Depreciation	22
Tax Cuts and Jobs Act of 2017, Proposed Low Income Program Funding from TCJA Benefits	2
Tax Cuts and Jobs Act of 2017, Proposed 2018 Natural Gas Refund of TCJA Benefits	$ 3