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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - PUCT - TNMP - USD ($) $ in Millions | Jul. 23, 2021 | Mar. 12, 2021 | Oct. 07, 2020 | Mar. 27, 2020
--- | --- | --- | --- | ---
Public Utilities, General Disclosures [Line Items] |  |  |  |
Approved Increase in Rate Base |  | $ 112.6 | $ 10.8 | $ 59.2
Annual Increase in Revenue |  | $ 14.1 | $ 2.0 | $ 7.8
Subsequent event |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Approved Increase in Rate Base | $ 41.2 |  |  |
Annual Increase in Revenue | $ 6.3 |  |  |

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