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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - TNMP - USD ($) $ in Millions | Mar. 27, 2018 | Sep. 13, 2017 | Mar. 14, 2017 | Sep. 08, 2016 | Mar. 23, 2016
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Transmission Cost of Service Rates |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Approved Increase in Rate Base |  |  | $ 30.2 | $ 9.5 | $ 25.8
Annual Increase in Revenue |  |  | $ 4.8 | $ 1.8 | $ 4.3
PUCT |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |
Requested rate increase (decrease) | $ 32.0 | $ 27.5 |  |  |
Proposed increase (decrease) in revenues | $ 0.6 | $ 4.7 |  |  |

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