Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

9 Months Ended

Sep. 30, 2020

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
	September 30, 2020						September 30, 2019
	Electric		Natural Gas		Total		Electric		Natural Gas		Total
Montana	$	78.5	$	9.9	$	88.4	$	68.4	$	8.9	$	77.3
South Dakota	18.9		1.7		20.6		16.0		1.7		17.7
Nebraska	—		1.7		1.7		—		1.8		1.8
Residential	97.4		13.3		110.7		84.4		12.4		96.8
Montana	89.1		5.6		94.7		87.7		5.5		93.2
South Dakota	27.4		1.0		28.4		26.3		1.3		27.6
Nebraska	—		0.7		0.7		—		0.9		0.9
Commercial	116.5		7.3		123.8		114.0		7.7		121.7
Industrial	9.2		0.1		9.3		10.6		0.1		10.7
Lighting, Governmental, Irrigation, and Interdepartmental	11.9		0.1		12.0		12.4		0.1		12.5
Total Customer Revenues	235.0		20.8		255.8		221.4		20.3		241.7
Other Tariff and Contract Based Revenues	14.9		8.3		23.2		15.3		7.7		23.0
Total Revenue from Contracts with Customers	249.9		29.1		279.0		236.7		28.0		264.7
Regulatory amortization	(5.7)		7.4		1.7		4.5		5.6		10.1
Total Revenues	$	244.2	$	36.5	$	280.7	$	241.2	$	33.6	$	274.8

	Nine Months Ended
	September 30, 2020						September 30, 2019
	Electric		Natural Gas		Total		Electric		Natural Gas		Total
Montana	$	237.7	$	65.7	$	303.4	$	225.4	$	73.3	$	298.7
South Dakota	52.4		16.7		69.1		47.4		20.4		67.8
Nebraska	—		12.9		12.9		—		15.7		15.7
Residential	290.1		95.3		385.4		272.8		109.4		382.2
Montana	252.5		33.0		285.5		257.3		38.0		295.3
South Dakota	77.1		11.2		88.3		71.2		14.1		85.3
Nebraska	—		6.2		6.2		—		8.3		8.3
Commercial	329.6		50.4		380.0		328.5		60.4		388.9
Industrial	27.2		0.5		27.7		32.4		0.7		33.1
Lighting, Governmental, Irrigation, and Interdepartmental	26.4		0.7		27.1		25.2		0.6		25.8
Total Customer Revenues	673.3		146.9		820.2		658.9		171.1		830.0
Other Tariff and Contract Based Revenues	44.1		26.5		70.6		46.5		26.6		73.1
Total Revenue from Contracts with Customers	717.4		173.4		890.8		705.4		197.7		903.1
Regulatory amortization	(10.7)		5.1		(5.6)		28.5		(1.9)		26.6
Total Revenues	$	706.7	$	178.5	$	885.2	$	733.9	$	195.8	$	929.7