Schedule I - Condensed Financial Information of Parent Company - Balance Sheets Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Condensed Financial Statements, Captions [Line Items]
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	85,834,874	85,834,874
Common stock, shares outstanding (in shares)	79,653,624	79,653,624
PNM Resources
Condensed Financial Statements, Captions [Line Items]
Accumulated depreciation	$ 15,706	$ 14,583
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	120,000,000	120,000,000
Common stock, shares issued (in shares)	85,834,874	85,834,874
Common stock, shares outstanding (in shares)	85,834,874	85,834,874