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Summary of the Business and Significant Accounting Policies - Narrative (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Jan. 01, 2017 USD ($) | Dec. 31, 1986 lease
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Payment defaults | $ 0 |  |  |  |
Unrealized gains, net of income taxes, recorded in AOCI | 3,800,000 |  |  |  |
Allowance for equity funds used during construction | $ 4,949,000 | $ 10,430,000 | $ 5,563,000 |  |
Weighted average borrowing rate | 6.10% | 6.60% | 6.60% |  |
Interest costs incurred, capitalized | $ 1,800,000 | $ 1,500,000 | $ 1,600,000 |  |
Subsequent Event [Member] | Accounting Standards Update 2016-09 [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Reduction of accumulated deferred income tax liabilities |  |  |  | $ 8,400,000 |
Subsequent Event [Member] | Accounting Standards Update 2016-09 [Member] | Retained Earnings [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Increase in retained earnings |  |  |  | $ 8,400,000 |
Public Service Company of New Mexico [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Allowance for funds used during construction, capitalized interest | 5,300,000 | 7,800,000 | 4,200,000 |  |
Allowance for equity funds used during construction | 4,163,000 | 10,430,000 | 5,563,000 |  |
Interest costs incurred, capitalized | 800,000 | 800,000 | 1,100,000 |  |
Impairment losses on securities held in the NDT | $ 13,900,000 | 10,400,000 | 4,800,000 |  |
Public Service Company of New Mexico [Member] | 10.3% Lessor Notes [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Notes receivable, stated percentage rate | 10.30% |  |  |  |
Public Service Company of New Mexico [Member] | 10.15% Lessor Notes [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Notes receivable, stated percentage rate | 10.15% |  |  |  |
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Number of operating leases | lease |  |  |  |  | 11
Texas-New Mexico Power Company [Member] |  |  |  |  |
Accounting Policies Disclosures [Line Items] |  |  |  |  |
Allowance for funds used during construction, capitalized interest | $ 900,000 | 500,000 | 500,000 |  |
Allowance for equity funds used during construction | 800,000 | 0 | 0 |  |
Interest costs incurred, capitalized | $ 100,000 | $ 100,000 | $ 100,000 |  |

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