Fair Value of Derivative and Other Financial Instruments - Maturities of Debt Securities (Details) - PNMR and PNM $ in Thousands	Mar. 31, 2019 USD ($)
Available-for-Sale
Within 1 year	$ 13,996
After 1 year through 5 years	68,108
After 5 years through 10 years	68,904
After 10 years through 15 years	9,485
After 15 years through 20 years	12,174
After 20 years	45,695
Available-for-sale debt securities	$ 218,362

Available-for-sale Securities, Debt Maturities, after Fifteen Years Through Twenty Years, Fair Value

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Available-for-sale Securities, Debt Maturities, after Ten Years Through Fifteen Years, Fair Value

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Available-for-sale Securities, Debt Maturities, after Twenty Years, Fair Value

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in sixth through tenth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), maturing in next fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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