Regulatory and Rate Matters - TNMP Narrative (Details) - TNMP $ in Millions			1 Months Ended		9 Months Ended	12 Months Ended
	Dec. 20, 2018 USD ($)	May 30, 2018 USD ($)	Oct. 31, 2019 USD ($)	Aug. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2017 USD ($)	Mar. 30, 2020 MWh	Dec. 31, 2019 USD ($)	Aug. 30, 2019 USD ($)	Mar. 01, 2019 USD ($)
Public Utilities, Commitments And Contingencies [Line Items]
Net regulatory liability						$ 37.8
Energy efficiency cost recovery, requested change amount									$ 5.9	$ 5.6
Energy efficiency cost recovery, requested bonus									$ 0.8	$ 0.8
Rider charge from electricity relief program (per MWh) | MWh							0.33
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
Subsequent event | Forecast
Public Utilities, Commitments And Contingencies [Line Items]
Proceeds from short-term debt					$ 0.5