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
	Mar. 15, 2017	Sep. 09, 2016	Mar. 22, 2016	Sep. 09, 2015	Mar. 15, 2015
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base		$ 25.8	$ 7.0	$ 27.1	$ 25.2
Annual Increase in Revenue		$ 4.3	$ 1.4	$ 4.4	$ 4.2
Subsequent event [Member]
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base	$ 9.5
Annual Increase in Revenue	$ 1.8