Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP $ in Millions					1 Months Ended		12 Months Ended
	Jun. 26, 2020 USD ($)	Apr. 14, 2020 USD ($)	Dec. 20, 2018 USD ($)	May 30, 2018 USD ($)	Oct. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Jul. 27, 2020 USD ($)	Jun. 30, 2020 USD ($)	May 29, 2020 USD ($)	Mar. 30, 2020 MWh	Mar. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Public Utilities, Commitments And Contingencies [Line Items]
Net regulatory liability							$ 37.8
Annual distribution revenue requirement	$ 14.3
Energy efficiency cost recovery, requested change amount										$ 5.9		$ 5.9
Energy efficiency cost recovery, requested bonus										$ 1.0		$ 0.8
Rider charge from electricity relief program (per MWh) | MWh											0.33
Proceeds from short-term debt		$ 0.5
COVID-19
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory liability									$ 0.4
Other COVID-19 related costs									$ 0.3
Forecast
Public Utilities, Commitments And Contingencies [Line Items]
Energy efficiency cost recovery, requested bonus								$ 1.0
2018 TNMP Rate Case
Public Utilities, Commitments And Contingencies [Line Items]
Requested rate increase (decrease)			$ 10.0	$ 25.9
Requested return on equity			9.65%	10.50%
Requested cost of debt			6.44%	7.20%
Requested debt capital structure			55.00%	50.00%
Requested equity capital structure			45.00%	50.00%
Investments excluded from rate request			$ 10.6
Refund of federal income tax rates period							5 years
Recovery of costs incurred					$ 3.3	$ 3.8
Recovery costs collection period						3 years
Write off of regulatory disallowance													$ 0.5