Regulatory and Rate Matters - Renewable Energy Rider (Details) - Public Service Company of New Mexico - Renewable energy rider - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Public Utilities, General Disclosures [Line Items]
Revenue from renewable energy rider	$ 52.0	$ 41.4	$ 45.2
Maximum
Public Utilities, General Disclosures [Line Items]
Annual revenue to be collected	$ 58.9
NMPRC-approved return on equity	0.50%