Significant Accounting Policies and Responsibility for Financial Statements (Details) - USD ($)	1 Months Ended				3 Months Ended		9 Months Ended
	Sep. 30, 2017	Jul. 31, 2017	Sep. 30, 2016	Jul. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Payment defaults under agreements	$ 0				$ 0		$ 0
Unrealized gains, net of income taxes, recorded in AOCI	$ 9,800,000				$ 9,800,000		$ 9,800,000
Dividends Payable [Line Items]
Declared dividends on common stock (dollars per share)	$ 0.2425	$ 0.2425	$ 0.22	$ 0.22	$ 0.2425	$ 0.22	$ 0.7275	$ 0.66
Dividends declared and paid							$ 57,948,000
PNM
Dividends Payable [Line Items]
Equity contribution								$ 28,100,000
Dividends declared and paid								4,100,000
Texas-New Mexico Power Company
Dividends Payable [Line Items]
Equity contribution								50,000,000
Dividends declared and paid							$ 29,663,000	$ 18,000,000