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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - TNMP - USD ($) $ in Millions | Apr. 06, 2020 | Mar. 27, 2020 | Sep. 19, 2019 | Mar. 21, 2019
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Subsequent event |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Requested increase in annual distribution revenue | $ 14.7 |  |  |
Incremental distribution investments | $ 149.2 |  |  |
PUCT |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |
Approved Increase in Rate Base |  | $ 59.2 | $ 21.9 | $ 111.8
Annual Increase in Revenue |  | $ 7.8 | $ 3.3 | $ 14.3

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