Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP $ in Millions						1 Months Ended		12 Months Ended
	Jun. 26, 2020 USD ($)	Apr. 14, 2020 USD ($)	Apr. 06, 2020 USD ($)	Dec. 20, 2018 USD ($)	May 30, 2018 USD ($)	Oct. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Oct. 02, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jul. 27, 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
Requested increase in annual distribution revenue			$ 14.7
Incremental distribution investments			$ 149.2
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	$ 1.0		$ 0.8
Rider charge from electricity relief program (per MWh) | MWh													0.33
Proceeds from short-term debt		$ 0.5
Subsequent event
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, estimate of possible upgrades									$ 46.0
COVID-19
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory liability										$ 0.5
Other COVID-19 related costs										$ 0.5
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