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Lease Commitments (Details) - PNM $ in Millions | Jan. 15, 2016 USD ($) lease MW | Sep. 30, 2017 USD ($) lease | Jan. 31, 2016 lease | Jan. 15, 2015 lease
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Palo Verde Nuclear Generating Station, Unit 1 Leases |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Number of leases, expiring |  |  |  | 4
Number of leases under which lease term was extended |  |  |  | 4
Palo Verde Nuclear Generating Station, Unit 2 Leases |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Number of leases, expiring | 4 |  |  |
Number of leases under which lease term was extended | 1 |  |  |
Number of leases under which assets were purchased | 3 | 3 | 3 |
Leased capacity to be purchased (in megawatts) | MW | 32.8 |  |  |
Palo Verde Nuclear Generating Station, Unit 2 Leases, 31.25 MW |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Number of leases under which assets were purchased | 1 |  |  |
Payment to lessors | $ | $ 78.1 |  |  |
Palo Verde Nuclear Generating Station, Unit 2 Leases, January 15, 2016 |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Leased capacity to be purchased (in megawatts) | MW | 31.3 |  |  |
Palo Verde Nuclear Generating Station, Unit 2 Leases, 32.76 MW |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Number of leases under which assets were purchased | 2 |  |  |
Payment to lessors | $ | $ 85.2 |  |  |
Palo Verde Nuclear Generating Station |  |  |  |
Operating Leased Assets [Line Items] |  |  |  |
Loss contingency, lease arrangements (up to) | $ |  | $ 169.9 |  |

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