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Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP $ in Millions					1 Months Ended
	Apr. 05, 2021 USD ($)	Jun. 26, 2020 USD ($)	Apr. 14, 2020 USD ($)	Apr. 06, 2020 USD ($)	Oct. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)	Mar. 31, 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				$ 14.7
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				$ 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
Subsequent event
Public Utilities, Commitments And Contingencies [Line Items]
Requested increase in annual distribution revenue	$ 14.0
Increase in revenue base rate	$ 104.5
COVID-19
Public Utilities, Commitments And Contingencies [Line Items]
Regulatory liability							$ 0.1		$ 0.1
Other COVID-19 related costs							$ 0.8		$ 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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