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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
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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