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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - Texas-New Mexico Power Company - Transmission Cost of Service Rates - USD ($) $ in Millions	Mar. 14, 2017	Sep. 08, 2016	Mar. 23, 2016	Sep. 10, 2015
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base	$ 30.2	$ 9.5	$ 25.8	$ 7.0
Annual Increase in Revenue	$ 4.8	$ 1.8	$ 4.3	$ 1.4

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- Definition

Public Utilities, Increase Annual Transmission Service Revenue

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Amount of public utility's approved rate increase (decrease) by regulatory agency.

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