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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - TNMP - USD ($) $ in Millions	Jul. 23, 2019	Mar. 21, 2019	Mar. 27, 2018	Sep. 13, 2017	Mar. 14, 2017
Transmission Cost of Service Rates
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base	$ 21.9				$ 30.2
Annual Increase in Revenue	$ 3.3				$ 4.8
PUCT
Public Utilities, General Disclosures [Line Items]
Requested rate increase (decrease)		$ 111.8	$ 32.0	$ 27.5
Proposed increase (decrease) in revenues		$ 14.3	$ 0.6	$ 4.7

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