Qualifying Cash Flow Hedges (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1		$ (20.2)		$ (20.9)		$ (18.2)
After-tax net unrealized gains (losses) related to derivatives accounted for as hedges		0.0		0.0		0.1
After-tax net realized losses on derivative transactions reclassified into earnings		0.8		0.7		0.9
Adoption of ASU. 2018-02 (a)	[1]	0.0		0.0		(3.7)
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31		(19.4)		(20.2)		(20.9)
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Fair Value Hedges, Net		0.0		0.0		0.0
Not Designated as Hedging Instrument
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(1.4)		(3.7)		7.4
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)		(1.7)		(1.4)		(12.8)
Derivative, Gain (Loss) on Derivative, Net		(3.2)		9.6		8.2
Not Designated as Hedging Instrument | Commodity Trading Contract
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)		0.0		0.0		0.0
Derivative, Gain (Loss) on Derivative, Net	[2]	(0.7)		10.9		9.4
Not Designated as Hedging Instrument | Electric Commodity Contract
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		1.5		(5.5)		9.3
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	[3]	(0.8)		(3.3)		(13.8)
Derivative, Gain (Loss) on Derivative, Net		0.0		0.0		0.0
Not Designated as Hedging Instrument | Natural Gas Commodity Contract
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities		(2.9)		1.8		(1.9)
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		0.0		0.0		0.0
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)	[4]	(0.9)		(1.9)		(1.0)
Derivative, Gain (Loss) on Derivative, Net	[4]	(2.5)		(1.3)		(1.2)
Designated as Hedging Instrument | Cash Flow Hedges
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		1.1		1.0		(0.1)
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)		0.0		0.0		0.0
Derivative, Gain (Loss) on Derivative, Net		0.0		0.0
Designated as Hedging Instrument | Cash Flow Hedges | Interest Rate Contract
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss		1.1	[5]	1.1	[5]	1.5	[6]
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)		0.0		0.0		0.0
Derivative, Gain (Loss) on Derivative, Net		$ 0.0		0.0		0.0
Designated as Hedging Instrument | Cash Flow Hedges | Vehicle Fuel Commodity
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss				(0.1)	[6]	(0.1)
Pre-tax gains (losses) reclassified into income during the period from regulatory assets and (liabilities)				0.0		$ 0.0
Derivative, Gain (Loss) on Derivative, Net				$ 0.0

[1] In 2017, NSP-Minnesota implemented ASU No. 2018-02 related to TCJA, which resulted in reclassification of certain credit balances within net accumulated other comprehensive loss to retained earnings.

[2] Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.

[3] Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.

[4] Amounts are recorded to cost of natural gas sold and transported. These derivative settlement gains and losses are shared with natural gas customers through purchased natural gas cost-recovery mechanisms, and reclassified out of income as regulatory assets and liabilities, as appropriate.

[5] Amounts are recorded to interest charges.

[6] Amounts are recorded to O&M expenses.