Benefit Plans and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2019

Benefit Plans and Other Postretirement Benefits [Abstract]

Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost

	Pension Benefits						Postretirement Benefits
(Millions of Dollars)	2019			2018			2019			2018
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost:
Net loss	$	440.0		$	502.0		$	36.8		$	34.3
Prior service credit	(0.2		)	(1.2		)	(9.4		)	(12.4		)
Total	$	439.8		$	500.8		$	27.4		$	21.9
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates:
Current regulatory assets	$	29.3		$	35.5		$	—		$	—
Noncurrent regulatory assets	410.5			465.3			25.6			20.5
Deferred income taxes	—			—			0.5			0.4
Net-of-tax accumulated other comprehensive income	—			—			1.3			1.0
Total	$	439.8		$	500.8		$	27.4		$	21.9
Measurement date	Dec. 31, 2019			Dec. 31, 2018			Dec. 31, 2019			Dec. 31, 2018

Projected Benefit Payments for the Pension and Postretirement Benefit Plans

Projected Benefit Payments

NSP-Minnesota’s projected benefit payments:

(Millions of Dollars)	Projected Pension Benefit Payments		Gross Projected Postretirement Health Care Benefit Payments		Expected Medicare Part D Subsidies		Net Projected Postretirement Health Care Benefit Payments
2020	$	89.9	$	6.9	$	—	$	6.9
2021	82.4		6.6		—		6.6
2022	80.8		6.2		—		6.2
2023	78.5		5.9		—		5.9
2024	74.1		5.6		—		5.6
2025-2029	326.7		23.6		—		23.6

Pension Plan

Benefit Plans and Other Postretirement Benefits [Abstract]

Target Asset Allocations and Plan Assets Measured at Fair Value

or each of the fair value hierarchy levels, NSP-Minnesota’s pension plan assets measured at fair value:

	Dec. 31, 2019 (a)													Dec. 31, 2018 (a)
(Millions of Dollars)	Level 1			Level 2		Level 3		Measured at NAV			Total			Level 1		Level 2		Level 3		Measured at NAV			Total
Cash equivalents	$	40.9		$	—	$	—	$	—		$	40.9		$	31.8	$	—	$	—	$	—		$	31.8
Commingled funds	360.3			—		—		270.3			630.6			241.0		—		—		271.2			512.2
Debt securities	—			155.5		1.1		—			156.6			—		143.7		—		—			143.7
Equity securities	22.6			—		—		—			22.6			29.3		—		—		—			29.3
Other	(31.5		)	1.2		—		(5.2		)	(35.5		)	0.5		1.3		—		(8.2		)	(6.4		)
Total	$	392.3		$	156.7	$	1.1	$	265.1		$	815.2		$	302.6	$	145.0	$	—	$	263.0		$	710.6

^(a)  See Note 8 for further information on fair value measurement inputs and methods.

For each of the fair value hierarchy levels, NSP-Minnesota’s postretirement benefit plan assets that were measured at fair value:

	Dec. 31, 2019 (a)										Dec. 31, 2018 (a)
(Millions of Dollars)	Level 1		Level 2		Level 3		Measured at NAV		Total		Level 1		Level 2		Level 3		Measured at NAV		Total
Cash equivalents	$	0.1	$	—	$	—	$	—	$	0.1	$	0.1	$	—	$	—	$	—	$	0.1
Insurance contracts	—		0.3		—		—		0.3		—		0.3		—		—		0.3
Commingled funds	0.4		—		—		0.5		0.9		0.8		—		—		0.2		1.0
Debt securities	—		1.3		—		—		1.3		—		1.0		—		—		1.0
Equity securities	—		—		—		—		—		—		—		—		—		—
Total	$	0.5	$	1.6	$	—	$	0.5	$	2.6	$	0.9	$	1.3	$	—	$	0.2	$	2.4

Changes in Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status of Plan [Table Text Block]

	Pension Benefits						Postretirement Benefits
(Millions of Dollars)	2019			2018			2019			2018
Change in Benefit Obligation:
Obligation at Jan. 1	$	907.0		$	1,035.1		$	76.0		$	88.8
Service cost	25.4			28.0			0.1			0.2
Interest cost	37.1			35.2			3.2			3.1
Plan amendments	1.0			—			—			—
Actuarial loss (gain)	61.7			(50.8		)	3.8			(9.0		)
Plan participants’ contributions	—			—			0.3			0.4
Benefit payments (a)	(90.0		)	(140.5		)	(7.9		)	(7.5		)
Obligation at Dec. 31	$	942.2		$	907.0		$	75.5		$	76.0
Change in Fair Value of Plan Assets:
Fair value of plan assets at Jan. 1	$	710.6		$	824.5		$	2.4		$	6.5
Actual return on plan assets	147.8			(36.5		)	—			—
Employer contributions	46.8			63.1			7.7			3.0
Plan participants’ contributions	—			—			0.3			0.4
Benefit payments	(90.0		)	(140.5		)	(7.8		)	(7.5		)
Fair value of plan assets at Dec. 31	$	815.2		$	710.6		$	2.6		$	2.4
Funded status of plans at Dec. 31	$	(127.0	)	$	(196.4	)	$	(72.9	)	$	(73.6	)
Amounts recognized in the Consolidated Balance Sheet at Dec. 31:
Current assets (liabilities)	$	—		$	—		$	(4.3	)	$	(4.8	)
Noncurrent assets (liabilities)	(127.0		)	(196.4		)	(68.6		)	(68.8		)
Net amounts recognized	$	(127.0	)	$	(196.4	)	$	(72.9	)	$	(73.6	)

    

^(a)  Includes approximately $105 million of lump-sum benefit payments used in the determination of a settlement charge in 2018.

Significant Assumptions Used to Measure Benefit Obligations:
Discount rate for year-end valuation	3.49	%	4.31	%	3.47	%	4.32	%
Expected average long-term increase in compensation level	3.75	%	3.75	%	N/A		N/A
Mortality table	Pri-2012		RP-2014		Pri-2012		RP-2014
Health care costs trend rate — initial: Pre-65	N/A		N/A		6.00	%	6.50	%
Health care costs trend rate — initial: Post-65	N/A		N/A		5.10	%	5.30	%
Ultimate trend assumption — initial: Pre-65	N/A		N/A		4.50	%	4.50	%
Ultimate trend assumption — initial: Post-65	N/A		N/A		4.50	%	4.50	%
Years until ultimate trend is reached	N/A		N/A		3		4

Components of Net Periodic Benefit Costs

	Pension Benefits									Postretirement Benefits
(Millions of Dollars)	2019			2018			2017			2019			2018			2017
Service cost	$	25.4		$	28.0		$	27.8		$	0.1		$	0.2		$	0.1
Interest cost	37.1			35.2			40.7			3.2			3.1			3.4
Expected return on plan assets	(54.3		)	(58.2		)	(60.1		)	(0.1		)	(0.4		)	(0.2		)
Amortization of prior service cost	(0.1		)	(0.1		)	1.1			(3.0		)	(3.0		)	(3.0		)
Amortization of net loss	30.2			38.5			39.6			1.5			2.4			2.0
Settlement charge (a)	—			48.8			48.2			—			—			—
Net periodic pension cost	38.3			92.2			97.3			1.7			2.3			2.3
Costs not recognized due to effects of regulation	(5.2		)	(66.0		)	(72.2		)	—			—			—
Net benefit cost recognized for financial reporting	$	33.1		$	26.2		$	25.1		$	1.7		$	2.3		$	2.3
Significant Assumptions Used to Measure Costs:
Discount rate	4.31		%	3.63		%	4.13		%	4.32		%	3.62		%	4.13		%
Expected average long-term increase in compensation level	3.75			3.75			3.75			—			—			—
Expected average long-term rate of return on assets	7.10			7.10			7.10			4.50			5.30			5.80

^(a) 

A settlement charge is required when the amount of lump-sum distributions during the year is greater than the sum of the service and interest cost components of the annual net periodic pension cost. In 2018 and 2017, as a result of lump-sum distributions during the 2018 and 2017 plan years, NSP-Minnesota recorded a total pension settlement charge of $48.8 million in 2018 and $48.2 million in 2017 ,

Postretirement Benefits Plan

Benefit Plans and Other Postretirement Benefits [Abstract]

Target Asset Allocations and Plan Assets Measured at Fair Value

Target asset allocations:

	Pension Benefits				Postretirement Benefits
	2019		2018		2019		2018
Domestic and international equity securities	37	%	37	%	15	%	18	%
Long-duration fixed income and interest rate swap securities	30		28		—		—
Short-to-intermediate fixed income securities	14		18		72		70
Alternative investments	17		15		9		8
Cash	2		2		4		4
Total	100	%	100	%	100	%	100	%