Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

6 Months Ended

Jun. 30, 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
	June 30, 2019							June 30, 2018
	Electric		Natural Gas			Total		Electric			Natural Gas			Total
Montana	$	62.9	$	18.8		$	81.7	$	59.5		$	17.6		$	77.1
South Dakota	13.4		5.6			19.0		14.4			5.6			20.0
Nebraska	—		4.2			4.2		—			5.0			5.0
Residential	76.3		28.6			104.9		73.9			28.2			102.1
Montana	82.9		9.5			92.4		79.6			8.8			88.4
South Dakota	21.7		3.6			25.3		22.3			3.6			25.9
Nebraska	—		2.1			2.1		—			2.4			2.4
Commercial	104.6		15.2			119.8		101.9			14.8			116.7
Industrial	10.2		0.1			10.3		10.7			0.2			10.9
Lighting, Governmental, Irrigation, and Interdepartmental	7.8		0.2			8.0		7.1			0.2			7.3
Total Customer Revenues	198.9		44.1			243.0		193.6			43.4			237.0
Other Tariff and Contract Based Revenues	15.0		8.7			23.7		17.8			10.6			28.4
Total Revenue from Contracts with Customers	213.9		52.8			266.7		211.4			54.0			265.4
Regulatory amortization	5.8		(1.7		)	4.1		(1.7		)	(1.9		)	(3.6		)
Total Revenues	$	219.7	$	51.1		$	270.8	$	209.7		$	52.1		$	261.8

	Six Months Ended
	June 30, 2019							June 30, 2018
	Electric		Natural Gas			Total		Electric			Natural Gas			Total
Montana	$	157.0	$	64.4		$	221.4	$	146.7		$	58.5		$	205.2
South Dakota	31.4		18.7			50.1		33.1			17.0			50.1
Nebraska	—		13.8			13.8		—			16.4			16.4
Residential	188.4		96.9			285.3		179.8			91.9			271.7
Montana	169.6		32.5			202.1		163.3			29.4			192.7
South Dakota	44.9		12.8			57.7		46.3			11.5			57.8
Nebraska	—		7.4			7.4		—			8.5			8.5
Commercial	214.5		52.7			267.2		209.6			49.4			259.0
Industrial	21.8		0.6			22.4		21.5			0.7			22.2
Lighting, Governmental, Irrigation, and Interdepartmental	12.9		0.7			13.6		12.1			0.7			12.8
Total Customer Revenues	437.6		150.9			588.5		423.0			142.7			565.7
Other Tariff and Contract Based Revenues	31.2		18.9			50.1		35.6			20.9			56.5
Total Revenue from Contracts with Customers	468.8		169.8			638.6		458.6			163.6			622.2
Regulatory amortization	23.9		(7.6		)	16.3		(10.5		)	(8.4		)	(18.9		)
Total Revenues	$	492.7	$	162.2		$	654.9	$	448.1		$	155.2		$	603.3