Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

3 Months Ended

Mar. 31, 2019

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, 2019							March 31, 2018
	Electric		Natural Gas			Total		Electric			Natural Gas			Total
Montana	$	94.1	$	45.6		$	139.7	$	87.2		$	40.9		$	128.1
South Dakota	18.0		13.1			31.1		18.7			11.4			30.1
Nebraska	—		9.6			9.6		—			11.4			11.4
Residential	112.1		68.3			180.4		105.9			63.7			169.6
Montana	86.7		23.0			109.7		83.6			20.6			104.2
South Dakota	23.2		9.2			32.4		24.0			7.9			31.9
Nebraska	—		5.3			5.3		—			6.1			6.1
Commercial	109.9		37.5			147.4		107.6			34.6			142.2
Industrial	11.6		0.5			12.1		10.8			0.5			11.3
Lighting, Governmental, Irrigation, and Interdepartmental	5.1		0.5			5.6		5.0			0.5			5.5
Total Customer Revenues	238.7		106.8			345.5		229.3			99.3			328.6
Other Tariff and Contract Based Revenues	16.2		10.2			26.4		17.8			10.3			28.1
Total Revenue from Contracts with Customers	254.9		117.0			371.9		247.1			109.6			356.7
Regulatory amortization	18.1		(5.8		)	12.3		(8.8		)	(6.4		)	(15.2		)
Total Revenues	$	273.0	$	111.2		$	384.2	$	238.3		$	103.2		$	341.5