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Regulatory and Rate Matters (Tables)
6 Months Ended
Jun. 30, 2021
Regulated Operations [Abstract]  
Schedule of Rate Increases for Transmission Costs The following sets forth TNMP’s recent interim transmission cost rate increases:
Effective DateApproved Increase in Rate BaseAnnual Increase in Revenue
(In millions)
March 27, 2020$59.2 $7.8 
October 7, 202010.8 2.0 
March 12, 2021112.6 14.1