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Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP $ in Millions						1 Months Ended		9 Months Ended
	Jul. 01, 2021 USD ($)	Apr. 05, 2021 USD ($)	Jun. 26, 2020 USD ($)	Apr. 14, 2020 USD ($)	Apr. 06, 2020 USD ($)	Oct. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2021 USD ($)	Jul. 28, 2021 USD ($)	May 27, 2021 USD ($)	Mar. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)	Oct. 02, 2020 USD ($)	Mar. 30, 2020 MWh	Mar. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Public Utilities, Commitments And Contingencies [Line Items]
Requested increase in annual distribution revenue		$ 14.0			$ 14.7
Incremental distribution investments					$ 149.2
Annual distribution revenue requirement			$ 14.3
Increase in revenue base rate		$ 104.5
Collection of increase in annual distribution revenues	$ 13.5
Energy efficiency cost recovery, requested change amount											$ 7.2	$ 5.9				$ 5.9
Energy efficiency cost recovery, requested bonus										$ 2.3	$ 2.3	$ 1.0				$ 0.8
Rider charge from electricity relief program (per MWh) | MWh															0.33
Proceeds from short-term debt				$ 0.5
Public utilities, estimate of possible upgrades														$ 46.0
PUCT
Public Utilities, Commitments And Contingencies [Line Items]
Public utilities, costs of deployment								$ 9.0
COVID-19
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory liability													$ 0.1
Other COVID-19 related costs									$ 0.9				$ 0.7
2018 TNMP Rate Case
Public Utilities, Commitments And Contingencies [Line Items]
Recovery of costs incurred						$ 3.3	$ 3.8
Recovery costs collection period							3 years
Write off of regulatory disallowance																	$ 0.5

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