Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

12 Months Ended

Dec. 31, 2021

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue [Table Text Block]

The following tables disaggregate our revenue for the twelve months ended by major source and customer class (in millions):

December 31, 2021	Electric		Natural Gas		Total
Montana	334.6		126.0		460.6
South Dakota	65.4		26.6		92.0
Nebraska	—		21.0		21.0
Residential	400.0		173.6		573.6
Montana	356.7		64.7		421.4
South Dakota	102.5		19.1		121.6
Nebraska	—		11.4		11.4
Commercial	459.2		95.2		554.4
Industrial	37.9		1.1		39.0
Lighting, Governmental, Irrigation, and Interdepartmental	32.1		1.4		33.5
Total Customer Revenues	929.2		271.3		1,200.5
Other Tariff and Contract Based Revenues	89.5		36.8		126.3
Total Revenue from Contracts with Customers	1,018.7		308.1		1,326.8
Regulatory amortization	33.5		12.0		45.5
Total Revenues	$	1,052.2	$	320.1	$	1,372.3

December 31, 2020	Electric		Natural Gas		Total
Montana	320.8		103.5		424.3
South Dakota	66.6		21.5		88.1
Nebraska	—		16.9		16.9
Residential	387.4		141.9		529.3
Montana	338.3		51.3		389.6
South Dakota	101.1		14.3		115.4
Nebraska	—		8.1		8.1
Commercial	439.4		73.7		513.1
Industrial	36.8		0.9		37.7
Lighting, Governmental, Irrigation, and Interdepartmental	31.8		0.9		32.7
Total Customer Revenues	895.4		217.4		1,112.8
Other Tariff and Contract Based Revenues	58.5		35.5		94.0
Total Revenue from Contracts with Customers	953.9		252.9		1,206.8
Regulatory amortization	(13.1)		5.0		(8.1)
Total Revenues	$	940.8	$	257.9	$	1,198.7

December 31, 2019	Electric		Natural Gas		Total
Montana	308.8		109.4		418.2
South Dakota	62.5		25.8		88.3
Nebraska	—		20.2		20.2
Residential	371.3		155.4		526.7
Montana	348.1		55.7		403.8
South Dakota	97.1		19.3		116.4
Nebraska	—		10.5		10.5
Commercial	445.2		85.5		530.7
Industrial	43.6		1.0		44.6
Lighting, Governmental, Irrigation, and Interdepartmental	30.6		1.0		31.6
Total Customer Revenues	890.7		242.9		1,133.6
Other Tariff and Contract Based Revenues	61.7		35.8		97.5
Total Revenue from Contracts with Customers	952.4		278.7		1,231.1
Regulatory amortization	28.8		(2.0)		26.8
Total Revenues	$	981.2	$	276.7	$	1,257.9