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Regulatory and Rate Matters (Tables)
9 Months Ended
Sep. 30, 2018
Regulated Operations [Abstract]  
Schedule of Rate Increases for Transmission Costs
The following sets forth TNMP’s recent interim transmission cost rate increases:
Effective Date
 
Approved Increase in Rate Base
 
Annual Increase in Revenue
 
 
(In millions)
September 8, 2016
 
$
9.5

 
$
1.8

March 14, 2017
 
30.2

 
4.8

September 13, 2017
 
27.5

 
4.7

March 27, 2018
 
32.0

 
0.6