Fair Value of Derivative and Other Financial Instruments - Maturities of Securities (Details) $ in Thousands	Dec. 31, 2017 USD ($)
PNM Resources
Held-to-Maturity
Within 1 year	$ 0
After 1 year through 5 years	66,588
After 5 years through 10 years	0
After 10 years through 15 years	0
After 15 years through 20 years	0
After 20 years	0
Held-to-maturity debt securities	66,588
PNMR and PNM
Available-for-Sale
Within 1 year	4,460
After 1 year through 5 years	32,693
After 5 years through 10 years	48,681
After 10 years through 15 years	5,934
After 15 years through 20 years	11,983
After 20 years	45,782
Available-for-sale debt securities	$ 149,533