Regulatory and Rate Matters - TNMP Schedules (Details) - Texas-New Mexico Power Company - USD ($) $ in Millions						6 Months Ended	12 Months Ended
	Sep. 13, 2017	Mar. 14, 2017	Sep. 08, 2016	Mar. 23, 2016	Sep. 10, 2015	Sep. 09, 2015	Feb. 28, 2019	Feb. 28, 2018	Feb. 28, 2017	Feb. 29, 2016
Energy efficiency costs
Public Utilities, General Disclosures [Line Items]
Aggregate Collection Amount									$ 6.0	$ 5.7
Performance Bonus									$ 0.7	$ 1.5
Energy efficiency costs | Scenario, forecast
Public Utilities, General Disclosures [Line Items]
Aggregate Collection Amount							$ 6.0	$ 6.0
Performance Bonus							$ 1.1	$ 0.8
Transmission Cost of Service Rates
Public Utilities, General Disclosures [Line Items]
Approved Increase in Rate Base	$ 27.5	$ 30.2	$ 9.5	$ 25.8	$ 7.0	$ 27.1
Annual Increase in Revenue	$ 4.7	$ 4.8	$ 1.8	$ 4.3	$ 1.4	$ 4.4