Revenue from Contracts with Customers Disaggregation of Revenue (Tables)

12 Months Ended

Dec. 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):

	Twelve Months Ended
	December 31, 2019							December 31, 2018
	Electric		Natural Gas			Total		Electric		Natural Gas			Total
Montana	$	308.8	$	109.4		$	418.2	$	287.3	$	102.7		$	390.0
South Dakota	62.5		25.8			88.3		64.2		25.4			89.6
Nebraska	—		20.2			20.2		—		23.4			23.4
Residential	371.3		155.4			526.7		351.5		151.5			503.0
Montana	348.1		55.7			403.8		329.6		51.7			381.3
South Dakota	97.1		19.3			116.4		94.0		18.0			112.0
Nebraska	—		10.5			10.5		—		11.9			11.9
Commercial	445.2		85.5			530.7		423.6		81.6			505.2
Industrial	43.6		1.0			44.6		42.6		1.2			43.8
Lighting, Governmental, Irrigation, and Interdepartmental	30.6		1.0			31.6		29.6		1.0			30.6
Total Customer Revenues	890.7		242.9			1,133.6		847.3		235.3			1,082.6
Other Tariff and Contract Based Revenues	61.7		35.8			97.5		65.4		39.2			104.6
Total Revenue from Contracts with Customers	952.4		278.7			1,231.1		912.7		274.5			1,187.2
Regulatory amortization	28.8		(2.0		)	26.8		8.4		(3.6		)	4.8
Total Revenues	$	981.2	$	276.7		$	1,257.9	$	921.1	$	270.9		$	1,192.0