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Pension and Other Postretirement Benefit Plans (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other (deductions) | $ 2,624,000 | $ 6,709,000 |  | $ 9,867,000 | $ 17,372,000
Other income | 3,735,000 | 6,275,000 |  | 12,000,000 | 14,626,000
PNM |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other (deductions) | 2,085,000 | 4,964,000 |  | 7,314,000 | 14,490,000
Other income | 2,137,000 | 3,762,000 |  | 6,821,000 | 10,270,000
PNM | Pension Plan |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Service cost | 0 | 0 |  | 0 | 0
Interest cost | 6,068,000 | 6,727,000 |  | 18,203,000 | 20,181,000
Expected return on plan assets | (8,672,000) | (8,451,000) |  | (26,014,000) | (25,352,000)
Amortization of net (gain) loss | 4,087,000 | 4,001,000 |  | 12,261,000 | 12,004,000
Amortization of prior service cost | (241,000) | (241,000) |  | (724,000) | (724,000)
Net Periodic Benefit Cost | 1,242,000 | 2,036,000 |  | 3,726,000 | 6,109,000
Contributions by employer |  |  |  | 0 | 0
Total expected employer contributions for future fiscal years | 0 |  |  | 0 |
Expected employer contributions in year 5 | 5,500,000 |  |  | $ 5,500,000 |
PNM | Pension Plan | Minimum |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Assumptions used calculating net periodic benefit cost, discount rate |  |  |  | 4.00% |
PNM | Pension Plan | Maximum |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Assumptions used calculating net periodic benefit cost, discount rate |  |  |  | 5.10% |
PNM | OPEB Plan |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Service cost | 21,000 | 24,000 |  | $ 62,000 | 72,000
Interest cost | 860,000 | 1,006,000 |  | 2,579,000 | 3,019,000
Expected return on plan assets | (1,353,000) | (1,308,000) |  | (4,061,000) | (3,923,000)
Amortization of net (gain) loss | 588,000 | 921,000 |  | 1,765,000 | 2,762,000
Amortization of prior service cost | (416,000) | (416,000) |  | (1,248,000) | (1,248,000)
Net Periodic Benefit Cost | (300,000) | 227,000 |  | (903,000) | 682,000
Contributions by employer |  |  |  | 0 | 0
PNM | Executive Retirement Program |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Service cost | 0 | 0 |  | 0 | 0
Interest cost | 155,000 | 174,000 |  | 467,000 | 523,000
Expected return on plan assets | 0 | 0 |  | 0 | 0
Amortization of net (gain) loss | 90,000 | 78,000 |  | 269,000 | 235,000
Amortization of prior service cost | 0 | 0 |  | 0 | 0
Net Periodic Benefit Cost | 245,000 | 252,000 |  | 736,000 | 758,000
Contributions by employer | 400,000 | 400,000 |  | 1,300,000 | 1,200,000
Total expected employer contributions for fiscal year | 1,600,000 |  |  | 1,600,000 |
Estimated employer contributions in Year 2-5 | 5,700,000 |  |  | 5,700,000 |
TNMP |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other (deductions) | 149,000 | 1,030,000 |  | 1,209,000 | 1,229,000
Other income | 1,300,000 | 2,258,000 |  | 4,276,000 | 3,621,000
TNMP | Pension Plan |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Service cost | 0 | 0 |  | 0 | 0
Interest cost | 656,000 | 722,000 |  | 1,969,000 | 2,165,000
Expected return on plan assets | (991,000) | (945,000) |  | (2,972,000) | (2,834,000)
Amortization of net (gain) loss | 272,000 | 231,000 |  | 816,000 | 692,000
Amortization of prior service cost | 0 | 0 |  | 0 | 0
Net Periodic Benefit Cost | (63,000) | 8,000 |  | (187,000) | 23,000
Contributions by employer |  |  |  | 0 | 0
Estimated employer contributions in Year 2-5 | 0 |  |  | $ 0 |
TNMP | Pension Plan | Minimum |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Assumptions used calculating net periodic benefit cost, discount rate |  |  |  | 4.00% |
TNMP | Pension Plan | Maximum |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Assumptions used calculating net periodic benefit cost, discount rate |  |  |  | 5.10% |
TNMP | OPEB Plan |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Service cost | 33,000 | 36,000 |  | $ 100,000 | 107,000
Interest cost | 119,000 | 139,000 |  | 358,000 | 417,000
Expected return on plan assets | (135,000) | (114,000) |  | (406,000) | (342,000)
Amortization of net (gain) loss | (56,000) | (20,000) |  | (170,000) | (60,000)
Amortization of prior service cost | 0 | 0 |  | 0 | 0
Net Periodic Benefit Cost | (39,000) | 41,000 |  | (118,000) | 122,000
Contributions by employer | 0 | 0 |  | 300,000 | 700,000
Estimated employer contributions in Year 2-5 | 1,400,000 |  |  | 1,400,000 |
TNMP | Executive Retirement Program |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Service cost | 0 | 0 |  | 0 | 0
Interest cost | 7,000 | 8,000 |  | 22,000 | 25,000
Expected return on plan assets | 0 | 0 |  | 0 | 0
Amortization of net (gain) loss | 4,000 | 2,000 |  | 11,000 | 7,000
Amortization of prior service cost | 0 | 0 |  | 0 | 0
Net Periodic Benefit Cost | 11,000 | 10,000 |  | 33,000 | 32,000
Total expected employer contributions for fiscal year | 100,000 |  |  | 100,000 |
Estimated employer contributions in Year 2-5 | 400,000 |  |  | 400,000 |
TNMP | Executive Retirement Program | Maximum |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Contributions by employer | 100,000 | 100,000 |  | 100,000 | 100,000
Accounting Standards Update 2017-07 |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other (deductions) |  | $ 2,100,000 |  |  | $ 6,400,000
Accounting Standards Update 2017-07 | PNM |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other (deductions) | 1,100,000 |  |  | 3,200,000 |
Non-service cost deferred as regulatory assets | 100,000 |  |  | 300,000 |
Accounting Standards Update 2017-07 | TNMP |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Other income | 100,000 |  |  | 300,000 |
Non-service cost deferred as regulatory liability | $ 100,000 |  |  | $ 100,000 |
Forecast | TNMP | OPEB Plan |  |  |  |  |
Components of Net Periodic Benefit Cost |  |  |  |  |
Contributions by employer |  |  | $ 0 |  |

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