Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

6 Months Ended

Jun. 30, 2025

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue [Table Text Block]

The following tables disaggregate our revenue by major source and customer class (in millions):

	Three Months Ended
	June 30, 2025						June 30, 2024
	Electric		Natural Gas		Total		Electric		Natural Gas		Total
Montana	$	81.8	$	18.0	$	99.8	$	86.0	$	18.9	$	104.9
South Dakota	16.2		5.6		21.8		15.4		5.9		21.3
Nebraska	—		4.5		4.5		—		3.8		3.8
Residential	98.0		28.1		126.1		101.4		28.6		130.0
Montana	93.9		10.4		104.3		99.7		10.7		110.4
South Dakota	27.8		3.9		31.7		26.3		3.7		30.0
Nebraska	—		2.4		2.4		—		2.0		2.0
Commercial	121.7		16.7		138.4		126.0		16.4		142.4
Industrial	9.9		0.1		10.0		11.3		0.2		11.5
Lighting, governmental, irrigation, and interdepartmental	9.4		0.3		9.7		8.6		0.3		8.9
Total Retail Revenues	239.0		45.2		284.2		247.3		45.5		292.8
Regulatory Amortization	10.3		5.2		15.5		(10.9)		3.7		(7.2)
Transmission	28.1		—		28.1		22.4		—		22.4
Transportation, wholesale and other	2.1		12.8		14.9		1.3		10.6		11.9
Total Revenues(1)	$	279.5	$	63.2	$	342.7	$	260.1	$	59.8	$	319.9

	Six Months Ended
	June 30, 2025						June 30, 2024
	Electric		Natural Gas		Total		Electric		Natural Gas		Total
Montana	$	196.8	$	69.4	$	266.2	$	203.4	$	67.5	$	270.9
South Dakota	38.5		21.2		59.7		34.7		19.5		54.2
Nebraska	—		17.7		17.7		—		14.3		14.3
Residential	235.3		108.3		343.6		238.1		101.3		339.4
Montana	190.9		37.2		228.1		201.2		35.8		237.0
South Dakota	57.1		15.1		72.2		54.1		13.0		67.1
Nebraska	—		9.8		9.8		—		8.2		8.2
Commercial	248.0		62.1		310.1		255.3		57.0		312.3
Industrial	20.0		0.6		20.6		23.0		0.6		23.6
Lighting, governmental, irrigation, and interdepartmental	14.0		0.8		14.8		13.3		0.9		14.2
Total Retail Revenues	517.3		171.8		689.1		529.7		159.8		689.5
Regulatory Amortization	38.0		(4.2)		33.8		25.5		10.6		36.1
Transmission	54.7		—		54.7		44.8		—		44.8
Transportation, wholesale and other	5.0		26.7		31.7		3.3		21.6		24.9
Total Revenues(1)	$	615.0	$	194.3	$	809.3	$	603.3	$	192.0	$	795.3

(1) Certain amounts in the prior period have been reclassified to conform with current period presentation. These reclassifications have no effect on the reported financial results.