Fair Value of Derivative and Other Financial Instruments - Maturities of Debt Securities (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Held-to-Maturity
Within 1 year	$ 0
After 1 year through 5 years	76,353
After 5 years through 10 years	0
Held-to-maturity debt securities	76,353
PNMR and PNM
Available-for-Sale
Within 1 year	3,913
After 1 year through 5 years	22,766
After 5 years through 10 years	25,456
After 10 years through 15 years	5,178
After 15 years through 20 years	10,692
After 20 years	41,614
Available-for-sale debt securities	$ 109,619