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Significant Accounting Policies and Responsibility for Financial Statements (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2020 | Jul. 31, 2020 | Sep. 30, 2019 | Jul. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Payment defaults under agreements | $ 0 |  |  |  | $ 0 |  | $ 0 |
Dividends declared per common share (dollars per share) | $ 0.3075 | $ 0.3075 | $ 0.290 | $ 0.290 | $ 0.3075 | $ 0.2900 | $ 0.9225 | $ 0.8700
Dividends declared on common stock |  |  |  |  | $ 48,987,000 | $ 46,200,000 | $ 73,480,000 | $ 69,299,000
PNM |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Dividends declared on common stock |  |  |  |  | 0 | 0 | 40,654,000 | 0
Texas-New Mexico Power Company |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Dividends declared on common stock |  |  |  |  | $ 16,174,000 | $ 15,367,000 | $ 34,613,000 | $ 30,178,000

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