Rate Matters, NSP-Minnesota (Details) - NSP-Minnesota - USD ($) $ in Thousands	1 Months Ended							3 Months Ended
	Sep. 30, 2016	Aug. 31, 2016	Jun. 30, 2016	Dec. 31, 2015	Nov. 30, 2015	Feb. 28, 2015	Nov. 30, 2013	Mar. 31, 2017
MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Number Of Years Rate Case Is Applicable For					3 years
Public Utilities, Requested Return on Equity, Percentage					10.00%
Public Utilities, Requested Equity Capital Structure, Percentage					52.50%
MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 194,600
Public Utilities, Requested Rate Increase (Decrease), Percentage					6.40%
Public Utilities, Requested Rate Base, Amount					$ 7,800,000
MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2017
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 52,100
Public Utilities, Requested Rate Increase (Decrease), Percentage					1.70%
Public Utilities, Requested Interim Rate Increase (Decrease), Amount					$ 44,900
Public Utilities, Requested Rate Base, Amount					7,700,000
MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2018
Rate Matters [Abstract]
Public Utilities, Requested Rate Increase (Decrease), Amount					$ 50,400
Public Utilities, Requested Rate Increase (Decrease), Percentage					1.70%
Public Utilities, Requested Rate Base, Amount					$ 7,700,000
FERC Proceeding, MISO ROE Complaint [Member]
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, Maximum Equity Capital Structure Percentage Allowed Per The Complaint							50.00%
Minnesota Public Utilities Commission | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016
Rate Matters [Abstract]
Public Utilities, Approved Interim Rate Increase (Decrease), Amount				$ 163,700
Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case
Rate Matters [Abstract]
Public Utilities, Number Of Years Rate Case Is Applicable For		4 years
Public Utilities, Requested Return on Equity, Percentage		9.20%
Public Utilities, Requested Equity Capital Structure, Percentage		52.50%
Public Utilities, Length of Stay-out Provision, In Years		4 years
Public Utilities, Rate Increase Under the Settlement		$ 184,970
Public Utilities, Increase (Decrease) Related to Sales True-up		59,750
Public Utilities, Rate Impact		244,720
Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2016
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement		74,990
Public Utilities, Increase (Decrease) Related to Sales True-up		59,950
Public Utilities, Rate Impact		134,940
Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2017
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement		59,860
Public Utilities, Increase (Decrease) Related to Sales True-up		0
Public Utilities, Rate Impact		59,860
Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2018
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement		0
Public Utilities, Increase (Decrease) Related to Sales True-up		0
Public Utilities, Rate Impact		0
Settlement Group | MPUC Proceeding - Minnesota 2016 Multi-Year Electric Rate Case, Rates 2019
Rate Matters [Abstract]
Public Utilities, Rate Increase Under the Settlement		50,120
Public Utilities, Increase (Decrease) Related to Sales True-up		(200)
Public Utilities, Rate Impact		$ 49,920
Administrative Law Judge | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties			9.70%
MPUC, NDPSC, SDPUC, and DOC | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties								8.81%
MISO TOs | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties								10.92%
FERC Staff | FERC Proceeding, MISO ROE Complaint [Member]
Rate Matters [Abstract]
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, Recommended By Third Parties								8.78%
Federal Energy Regulatory Commission (FERC) | FERC Proceeding, MISO ROE Complaint [Member]
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 Basis Point Adder, Approved								50
Public Utilities, ROE Applicable To Transmission Formula Rates In The MISO Region, with RTO Adder, Approved								10.82%