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Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - Texas-New Mexico Power Company [Member] - Transmission Cost of Service Rates [Member] - USD ($)
$ in Millions
 6 Months Ended
Sep. 09, 2016
Mar. 22, 2016
Sep. 09, 2015
Mar. 15, 2015
Public Utilities, General Disclosures [Line Items]        
Approved Increase in Rate Base   $ 7.0 $ 27.1 $ 25.2
Annual Increase in Revenue   $ 1.4 $ 4.4 $ 4.2
Subsequent Event [Member]        
Public Utilities, General Disclosures [Line Items]        
Approved Increase in Rate Base $ 25.8      
Annual Increase in Revenue $ 4.3