Regulatory and Rate Matters - Transmission Cost of Service Rates (Details) - Transmission Cost of Service Rates [Member] - Texas-New Mexico Power Company [Member] - USD ($) $ in Millions	6 Months Ended
	Mar. 09, 2016	Sep. 15, 2015	Mar. 15, 2015	Sep. 07, 2014	Mar. 12, 2014
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base		$ 27.1	$ 25.2	$ 18.2	$ 18.1
Annual Increase in Revenue		$ 4.4	$ 4.2	$ 2.9	$ 2.8
Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base	$ 7.0
Annual Increase in Revenue	$ 1.4