Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

3 Months Ended

Mar. 31, 2018

Revenue from Contract with Customer [Abstract]

Disaggregation of Revenue [Table Text Block]

The following table disaggregates our revenue by major source and customer class (in millions):

	Three Months Ended
	March 31, 2018									March 31, 2017
	Electric			Natural Gas			Total			Electric			Natural Gas			Total
Montana	$	87.2		$	40.9		$	128.1		$	90.8		$	43.8		$	134.6
South Dakota	18.7			11.4			30.1			17.3			10.8			28.1
Nebraska	—			11.4			11.4			—			9.0			9.0
Residential	105.9			63.7			169.6			108.1			63.6			171.7
Montana	83.6			20.6			104.2			88.1			21.9			110.0
South Dakota	24.0			7.9			31.9			22.4			7.4			29.8
Nebraska	—			6.1			6.1			—			4.9			4.9
Commercial	107.6			34.6			142.2			110.5			34.2			144.7
Industrial	10.8			0.5			11.3			10.9			0.5			11.4
Lighting, Governmental, and Irrigation	5.0			0.5			5.5			5.2			0.5			5.7
Total Customer Revenues	229.3			99.3			328.6			234.7			98.8			333.5
Other Tariff and Contract Based Revenues	17.8			10.3			28.1			37.4			11.1			48.5
Total Revenue from Contracts with Customers	247.1			109.6			356.7			272.1			109.9			382.0
Regulatory amortization	(8.8		)	(6.4		)	(15.2		)	(5.9		)	(8.8		)	(14.7		)
Total Revenues	$	238.3		$	103.2		$	341.5		$	266.2		$	101.1		$	367.3