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Fair Value of Derivative and Other Financial Instruments - Maturities of Debt Securities (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Held-to-Maturity
Within 1 year	$ 0
After 1 year through 5 years	57,486
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	57,486
PNMR and PNM
Available-for-Sale
Within 1 year	8,562
After 1 year through 5 years	53,094
After 5 years through 10 years	66,603
After 10 years through 15 years	9,735
After 15 years through 20 years	10,061
After 20 years	36,263
Available-for-sale debt securities	$ 184,318

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