Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

3 Months Ended

Mar. 31, 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
	March 31, 2020									March 31, 2019
	Electric			Natural Gas			Total			Electric		Natural Gas			Total
Montana	$	88.6		$	38.3		$	126.9		$	94.1	$	45.6		$	139.7
South Dakota	18.9			10.3			29.2			18.0		13.1			31.1
Nebraska	—			7.7			7.7			—		9.6			9.6
Residential	107.5			56.3			163.8			112.1		68.3			180.4
Montana	86.0			19.2			105.2			86.7		23.0			109.7
South Dakota	26.5			7.3			33.8			23.2		9.2			32.4
Nebraska	—			4.0			4.0			—		5.3			5.3
Commercial	112.5			30.5			143.0			109.9		37.5			147.4
Industrial	8.8			0.3			9.1			11.6		0.5			12.1
Lighting, Governmental, Irrigation, and Interdepartmental	5.3			0.3			5.6			5.1		0.5			5.6
Total Customer Revenues	234.1			87.4			321.5			238.7		106.8			345.5
Other Tariff and Contract Based Revenues	14.9			9.6			24.5			16.2		10.2			26.4
Total Revenue from Contracts with Customers	249.0			97.0			346.0			254.9		117.0			371.9
Regulatory amortization	(4.4		)	(6.4		)	(10.8		)	18.1		(5.8		)	12.3
Total Revenues	$	244.6		$	90.6		$	335.2		$	273.0	$	111.2		$	384.2