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Variable Interest Entities (Details) |  |  |  |  |  |  | 7 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |
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Feb. 01, 2017 USD ($) | Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Jul. 17, 2014 USD ($) | May 30, 2014 | Oct. 08, 2013 USD ($) | Jul. 17, 2014 USD ($) | Dec. 31, 2016 USD ($) lease investor MW | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Feb. 21, 2017 USD ($) | Jul. 19, 2016 USD ($) | May 31, 2016 MW | Apr. 30, 2016 USD ($) | Jan. 15, 2016 lease MW | Jan. 15, 2015 lease | Dec. 31, 1986 lease | Dec. 31, 1985 lease
Variable Interest Entity, Statement Of Operation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings attributable to non-controlling interest |  |  |  |  |  |  |  | $ 14,519,000 | $ 14,910,000 | $ 14,127,000 |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  |  |  |  |  | 378,039,000 | 385,570,000 |  |  |  |  |  |  |  |  |
Total assets |  |  |  |  |  |  |  | 6,471,080,000 | 6,009,328,000 | 5,790,237,000 |  |  |  |  |  |  |  |
Current liabilities |  |  |  |  |  |  |  | 805,108,000 | 641,120,000 |  |  |  |  |  |  |  |  |
Owners’ equity – non-controlling interest |  |  |  |  |  |  |  | $ 68,920,000 | 71,407,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of different institutional investors of trust lessors | investor |  |  |  |  |  |  |  | 5 |  |  |  |  |  |  |  |  |  |
Accrued lease payments |  |  |  |  |  |  |  | $ 26,497,000 |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Statement Of Operation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings attributable to non-controlling interest |  |  |  |  |  |  |  | 14,519,000 | 14,910,000 | 14,127,000 |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  |  |  |  |  | 314,245,000 | 357,757,000 |  |  |  |  |  |  |  |  |
Total assets |  |  |  |  |  |  |  | 4,867,546,000 | 4,599,344,000 |  |  |  |  |  |  |  |  |
Current liabilities |  |  |  |  |  |  |  | 457,361,000 | 316,921,000 |  |  |  |  |  |  |  |  |
Owners’ equity – non-controlling interest |  |  |  |  |  |  |  | 68,920,000 | 71,407,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Delta [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payment for fixed charges |  |  |  |  |  |  | $ 3,500,000 |  |  |  |  |  |  |  |  |  |  |
Payment for variable charges |  |  |  |  |  |  | 600,000 |  |  |  |  |  |  |  |  |  |  |
Payments to acquire businesses, net |  |  |  | $ 22,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Project financing debt, amount at closing |  |  |  | 14,600,000 |  |  | 14,600,000 |  |  |  |  |  |  |  |  |  |  |
Recorded plant in service |  |  |  | 58,100,000 |  |  | 58,100,000 |  |  |  |  |  |  |  |  |  |  |
Accumulated depreciation |  |  |  | 23,500,000 |  |  | 23,500,000 |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  | 3,600,000 |  |  | 3,600,000 |  |  |  |  |  |  |  |  |  |  |
Deferred charges |  |  |  | 3,400,000 |  |  | 3,400,000 |  |  |  |  |  |  |  |  |  |  |
Current liabilities |  |  |  | 300,000 |  |  | 300,000 |  |  |  |  |  |  |  |  |  |  |
Non-current regulatory liabilities |  |  |  | $ 3,400,000 |  |  | $ 3,400,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entities, Other financial obligations due to beneficial owner |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |
Variable interest entities, Exposure beyond contractual lease payments |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |
Variable interest entities, Additional rights to assets beyond use of leased assets |  |  |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Valencia [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts purchased (in megawatts) | MW |  |  |  |  |  |  |  | 158 |  |  |  |  |  |  |  |  |  |
Power plant total construction costs |  |  |  |  |  |  |  | $ 90,000,000 |  |  |  |  |  |  |  |  |  |
Payment for fixed charges |  |  |  |  |  |  |  | 19,300,000 | 19,200,000 | 19,100,000 |  |  |  |  |  |  |  |
Payment for variable charges |  |  |  |  |  |  |  | $ 1,100,000 | 1,600,000 | 1,200,000 |  |  |  |  |  |  |  |
Purchase price, percentage of the book value reduced by related indebtedness |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Purchase price, percentage of fair market value |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |
Number of days to agreement on fair market value |  |  |  |  |  | 60 days |  |  |  |  |  |  |  |  |  |  |  |
Purchase price as determined by appraisal process |  |  |  |  |  | $ 85,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Long term contract option to purchase, approximate approval period |  |  |  |  |  |  |  | 15 months |  |  |  |  |  |  |  |  |  |
Long term contract option to purchase, ownership percentage |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Number of years the operating licenses for plants were extended |  |  |  |  |  |  |  | 20 years |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Statement Of Operation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  |  |  |  |  | $ 20,371,000 | 20,687,000 | 20,247,000 |  |  |  |  |  |  |  |
Operating expenses |  |  |  |  |  |  |  | (5,852,000) | (5,777,000) | (6,120,000) |  |  |  |  |  |  |  |
Earnings attributable to non-controlling interest |  |  |  |  |  |  |  | 14,519,000 | 14,910,000 | $ 14,127,000 |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  |  |  |  |  | 2,551,000 | 2,588,000 |  |  |  |  |  |  |  |  |
Net property, plant and equipment |  |  |  |  |  |  |  | 66,947,000 | 69,784,000 |  |  |  |  |  |  |  |  |
Total assets |  |  |  |  |  |  |  | 69,498,000 | 72,372,000 |  |  |  |  |  |  |  |  |
Current liabilities |  |  |  |  |  |  |  | 578,000 | 965,000 |  |  |  |  |  |  |  |  |
Owners’ equity – non-controlling interest |  |  |  |  |  |  |  | $ 68,920,000 | 71,407,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Valencia [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Option to purchase a percentage of the plant or VIE (up to ) |  |  |  |  |  | 50.00% |  | 50.00% |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | PVNGS [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued lease payments |  |  |  |  |  |  |  | $ 8,300,000 | $ 18,400,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Delta [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of long term contract for purchase of electric power, fixed costs |  |  |  |  |  |  |  | 20 years |  |  |  |  |  |  |  |  |  |
NM Capital [Member] | Coal Supply [Member] | San Juan Generating Station [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Loan agreement |  | $ 125,000,000.0 | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Issuance in letters of credit |  |  | 30,300,000 |  |  |  |  |  |  |  |  | $ 30,300,000 |  | $ 40,000,000 |  |  |  |
NM Capital [Member] | Coal Supply [Member] | San Juan Coal Company, Westmoreland [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  | $ 125,000,000 |  |  |  |  | $ 95,000,000 |  |  |  |  |  |  |  |  |  |
Interest receivable |  |  |  |  |  |  |  | $ 1,300,000 |  |  |  |  |  |  |  |  |  |
NM Capital [Member] | Coal Supply [Member] | San Juan Coal Company, Westmoreland [Member] | Subsequent Event [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  |  |  |  |  |  |  |  |  | $ 85,400,000 |  |  |  |  |  |  |
Scheduled principal payment | $ 9,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Palo Verde Nuclear Generating Station, Unit 1 Leases [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of leases | lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  | 4
Palo Verde Nuclear Generating Station, Unit 2 Leases [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts purchased (in megawatts) | MW |  |  |  |  |  |  |  |  |  |  |  |  | 64.1 |  | 64.1 |  |  |
Number of leases | lease |  |  |  |  |  |  |  | 3 |  |  |  |  |  |  | 1 |  | 4 |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of leases under which assets were purchased | lease |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |

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