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Variable Interest Entities (Details) |  |  |  | 12 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 01, 2018 USD ($) | Feb. 01, 2016 USD ($) | Jan. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) lease investor MW | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Feb. 20, 2018 USD ($) | Jul. 19, 2016 USD ($) | May 31, 2016 MW | 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 |  |  |  | $ 15,017,000 | $ 14,519,000 | $ 14,910,000 |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  | 294,420,000 | 378,039,000 |  |  |  |  |  |  |  |
Total assets |  |  |  | 6,646,103,000 | 6,471,080,000 | 6,009,328,000 |  |  |  |  |  |  |
Current liabilities |  |  |  | 835,644,000 | 805,108,000 |  |  |  |  |  |  |  |
Owners’ equity – non-controlling interest |  |  |  | 66,195,000 | 68,920,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Statement Of Operation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Earnings attributable to non-controlling interest |  |  |  | 15,017,000 | 14,519,000 | 14,910,000 |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  | 243,631,000 | 314,245,000 |  |  |  |  |  |  |  |
Total assets |  |  |  | 4,921,563,000 | 4,867,546,000 |  |  |  |  |  |  |  |
Current liabilities |  |  |  | 218,496,000 | 457,361,000 |  |  |  |  |  |  |  |
Owners’ equity – non-controlling interest |  |  |  | $ 66,195,000 | 68,920,000 |  |  |  |  |  |  |  |
Number of different institutional investors of trust lessors | investor |  |  |  | 5 |  |  |  |  |  |  |  |  |
Accrued lease payments |  |  |  | $ 25,726,000 |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |
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 | Valencia |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts purchased (in megawatts) | MW |  |  |  | 158 |  |  |  |  |  |  |  |  |
Payment for fixed charges |  |  |  | $ 19,600,000 | 19,300,000 | 19,200,000 |  |  |  |  |  |  |
Payment for variable charges |  |  |  | $ 1,300,000 | 1,100,000 | 1,600,000 |  |  |  |  |  |  |
Purchase price, percentage of the book value reduced by related indebtedness |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Purchase price, percentage of fair market value |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Variable Interest Entity, Statement Of Operation [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues |  |  |  | $ 20,887,000 | 20,371,000 | 20,687,000 |  |  |  |  |  |  |
Operating expenses |  |  |  | (5,870,000) | (5,852,000) | (5,777,000) |  |  |  |  |  |  |
Earnings attributable to non-controlling interest |  |  |  | 15,017,000 | 14,519,000 | $ 14,910,000 |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Current assets |  |  |  | 2,688,000 | 2,551,000 |  |  |  |  |  |  |  |
Net property, plant and equipment |  |  |  | 64,109,000 | 66,947,000 |  |  |  |  |  |  |  |
Total assets |  |  |  | 66,797,000 | 69,498,000 |  |  |  |  |  |  |  |
Current liabilities |  |  |  | 602,000 | 578,000 |  |  |  |  |  |  |  |
Owners’ equity – non-controlling interest |  |  |  | $ 66,195,000 | 68,920,000 |  |  |  |  |  |  |  |
Number of years the operating licenses for plants were extended |  |  |  | 20 years |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Valencia | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Option to purchase a percentage of the plant or VIE (up to ) |  |  |  | 50.00% |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | PVNGS | Palo Verde Nuclear Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued lease payments |  |  |  |  | $ 8,300,000 |  |  |  |  |  |  |  |
NM Capital | Coal Supply | San Juan Generating Station |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Loan agreement |  | $ 125,000,000.0 | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |
Issuance in letters of credit |  |  | 30,300,000 |  |  |  |  | $ 30,300,000 |  |  |  |  |
NM Capital | Coal Supply | San Juan Coal Company, Westmoreland |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  | $ 125,000,000 | $ 56,600,000 |  |  |  |  |  |  |  |  |
Interest receivable |  |  |  | $ 1,000,000 |  |  |  |  |  |  |  |  |
NM Capital | Coal Supply | San Juan Coal Company, Westmoreland | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Amount outstanding |  |  |  |  |  |  | $ 51,000,000 |  |  |  |  |  |
Scheduled principal payment | $ 5,600,000 |  |  |  |  |  |  |  |  |  |  |  |
Palo Verde Nuclear Generating Station, Unit 1 Leases | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of leases | lease |  |  |  |  |  |  |  |  |  |  | 4 |  | 4
Palo Verde Nuclear Generating Station, Unit 2 Leases | Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Interest Entity [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts purchased (in megawatts) | MW |  |  |  |  |  |  |  |  | 64.1 | 64.1 |  |  |
Variable Interest Entity, Consolidated, Carrying Amount, Assets and Liabilities, Net [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of leases | lease |  |  |  | 3 |  |  |  |  |  | 1 |  | 4 |
Number of leases under which assets were purchased | lease |  |  |  |  |  |  |  |  |  | 3 |  |  |

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