Regulatory and Rate Matters - TNMP Schedules (Details) - Texas-New Mexico Power Company [Member] - USD ($) $ in Millions				6 Months Ended			12 Months Ended
	Sep. 08, 2016	Mar. 23, 2016	Sep. 10, 2015	Sep. 09, 2015	Mar. 15, 2015	Sep. 07, 2014	Feb. 28, 2018	Feb. 28, 2017	Feb. 29, 2016	Feb. 28, 2015
Energy Efficiency [Member]
Public Utilities, General Disclosures [Line Items]
Aggregate Collection Amount									$ 5.7	$ 5.6
Performance Bonus									$ 1.5	$ 0.7
Energy Efficiency [Member] | Scenario, Forecast [Member]
Public Utilities, General Disclosures [Line Items]
Aggregate Collection Amount							$ 6.0	$ 6.0
Performance Bonus							$ 0.8	$ 0.7
Transmission Cost of Service Rates [Member]
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base	$ 9.5	$ 25.8	$ 7.0	$ 27.1	$ 25.2	$ 18.2
Annual Increase in Revenue	$ 1.8	$ 4.3	$ 1.4	$ 4.4	$ 4.2	$ 2.9