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Significant Accounting Policies and Responsibility for Financial Statements (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Jul. 28, 2017 | Jul. 31, 2016 | Jun. 30, 2017 | Jun. 30, 2016 | Jun. 30, 2017 | Jun. 30, 2016
Accounting Policies [Abstract] |  |  |  |  |  |
Payment defaults under agreements |  |  | $ 0 |  | $ 0 |
Equity contribution |  |  |  |  |  | $ 4,100,000
Unrealized gains, net of income taxes, recorded in AOCI |  |  | $ 7,800,000 |  | $ 7,800,000 |
Dividends Payable [Line Items] |  |  |  |  |  |
Declared dividends on common stock (dollars per share) |  | $ 0.22 | $ 0.2425 | $ 0.220 | $ 0.4850 | $ 0.44
Dividends declared and paid |  |  |  |  | $ 19,316,000 |
PNM |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |
Dividends declared and paid |  |  |  |  |  | $ 4,100,000
Texas-New Mexico Power Company |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |
Dividends declared and paid |  |  |  |  | $ 17,459,000 | $ 7,500,000
Subsequent event |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |
Declared dividends on common stock (dollars per share) | $ 0.2425 |  |  |  |  |

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